So, you completely disagree that:
1) Maradona cheated in WC'86
I completely disagree with making it remotely relevant.
2) He's where he is in terms of legacy (top-3 of all time) due to WC'86
I completely disagree with the notion that you can even attempt to qualify the assessment of a player removing the greatest tournament performance of all time. Take 1970 away from Pelé and he struggles as well, even if that was essentially a farewell coronation. Personally I'm far more impressed by 1958 but I'm probably in a minority.
3) WC'86 is what put him above Platini and Zico (You yourself said: " I've mentioned before that pre-WC 86 you had kids wanting to be like Platini, Zico or Maradona. Within weeks, they just wanted to be Maradona.")
I said I agreed with your base case.
4) He was underwhelming in European competitions
Concerning point 4, because, you had an earlier response to one of my posts on European competitions, I just checked what his teams did in European competitions overall on transfermarkt (other than his UEFA Cup win in 1989) without isolating European Cup & his time at Napoli:
1983: eliminated by Austria Vienna in the QF (Cup Winners Cup)
1984: eliminated by United in the QF (Cup Winners Cup)
1986: eliminated by Toulouse in the 1st round (UEFA Cup)
1987: eliminated by Real in the 1st round (European Cup)
1989: eliminated by Werder Bremen in the 2nd round (UEFA Cup)
1990: eliminated by Spartak Moscow in the 2nd round (European Cup)
I'd say pretty disappointing compared to other all-time greats in continental tournaments.
A completely irrelevant point.
This is where you really really really should listen to those saying you can't construct an argument googling things and applying your biases but living through them and actually understanding them.
I don't mean to be nasty, you are just younger and used to acquiring knowledge second hand through that vast resource that is the internet. It is how it is, nothing right or wrong about it.
You say it was expected that Italian teams would make European finals, which is just plain wrong. Nothing to do with "winning an internet argument" either, but I'll lay it out for you chronologically:
1) Throughout the 70s and early 80s the strongest league in Europe was the Bundesliga, by a distance. Bayern was dominant, but nothing like today, it was chockful with fantastic sides and players taking turns at challenging them. It was the Germans that regularly made deep runs in European competitions (and Liverpool, who still lost Kevin Keegan to the German league).
2) In the first half of the 80s Juventus emerges as a superteam. It was basically world champions Italy with Boniek in stead of Conti and Platini (for me at least, the best at the time). For a contemporary parallely, think how that dominant Spain were Barca with no Messi. A well oiled machine, with a defined and very effective system + the cherry on the cake.
3) Even then, it should be noted that Juve played differently in Serie A and Europe. They needed to adapt their style for the cup/knockout format. Platini would drop deeper and play like Pirlo in Europe, while he would typically be more advanced and incisive in the league. He was absolutely perfect and custom made for that adaptation. Going back to the earlier parallel, imagine if Messi could be regular Messi in the league and turn into Xavi to pull the strings from midfield as and when needed. Juve didn't have Messi and Xavi, they just had Platini performing both roles. Of course he was the best player in the world, even if Maradona was more exciting and unplayable as an individual.
4) The only team that really competed with them domestically was Roma. They were nowhere near as fun to watch. A strong, resilient Liedholm/Eriksson side that were very hard to beat and in Falcao, Cerezo and Conti had the ingredients to turn draws into wins. Falcão is woefully underrated, absolute beast of a midfield general.
5) You have to bear in mind two things here: two points for a win (whoever lost the least was champion, it was all geared towards defending) and only two foreigners allowed (else Roma could have had Zico and that would have made them insanely good). Neither of those hold all the way to 1995, which is why your randomly picked "Italians in Europe" timeframe makes zero sense.
6) So far what you have is two great sides which also happened to be set up as great cup sides, nothing to do with Italian dominance. Not that they dominated anything, Juve won the EC once and nobody else did until Milan emerges. Furthermore, depth was nothing like it is today when we regularly yap about possible quadruples. Nobody managed a double throughout the 80s, your player pool was entirely domestic bar two imports and with Juve hoarding the NT and Milan/Inter picking up spare parts, the rest were scrambling to have a decent XI, let alone depth. Doing well in Europe systematically sacrificed the league, so much so that the Juve side that won it did a Liverpool 2005 and only qualified to European competitions through winning the EC.
7) That brings you to the point of incentives. You seem to look down at Spartak, Steaua, Red Star, certainly diss Galatasaray at one point. What you fail to see there is many of these were dominant domestically, much like PSG in Ligue 1 today. They lived for Europe. There was also this somewhat relevant geopolitical context which we can summarise as "the iron curtain". The state and their FAs were heavily invested in the importance of performing in Europe so some of these were almost the national team out there, much like Puskas' Honved were effectively Hungary 40 years earlier. At the other end you have the likes of Juve and Milan, which have won the scudetto plenty of times and happily sacrifice it for Europe. Napoli? They had never won it, they are a team from the deep historically impoverished south, they don't give two shits about Europe or geopolitics and they only live for the day they stick it up to the rich industrial northerners. Feck Europe, the league is all that matters. They didn't want to do a Verona and knick the league only to be 10th the next season.
8) You note the importance of World Cup 86 for Maradona, but not its importance and that of Maradona to Serie A. Serie A took off in the second half of the 80s. Money poured in and suddenly you had the Bundesliga losing their top players to it. Where Juve had been the base of the Italian team, now Milan was. The rich northerners wouldn't have these bloody southerners sticking it up to them so went shopping for the very best and since two wasn't enough they made it three foreigners, eventually 3+2 naturalised and later there was Bosman and it all went haywire. Again, it makes absolutely no sense to have pre-1988 compared to the 90s. Completely different constraints.
9) Suddenly you had Milan adding a spine of Rijkaard, Gullit and van Basten to their Italian NT core. Inter instead went for Matthäus, Brehme and Klinsmann. This is basically the biggest stars of two of the best NTs at the time added to the lion's share of the Italian NT. Napoli had Maradona, Careca and added Alemao to an Italian core with a handful of NT subs. Good players, mind, but not at the same world class tier as the others. I don't actually find the first Napoli scudetto all that remarkable. It could easily be marked as a Verona/Leicester one off. What was incredible was how even after that rule change in 1988 and others arming thenselves to the teeth, they managed to pick up a UEFA Cup and another Serie A. That was the truly remarkable club-level feat.
10) You mention how "Napoli's rivals" Milan managed to do well in Europe. Firstly, nobody in their right mind would compare the status and greatness of that AC side to the Napoli one, so using their relative accomplishments as an evaluation of Maradona is mental. Still, for all their legendary all-time status, they had to watch Inter and Napoli reclaim the league while they excelled in Europe. They won two ECs off a single league win. They were a great cup side which prioritised that, so instead of losing 2 league games they would end up losing 5 or 6 and that was the league gone.
I'll pass on going beyond 1990. It is completely irrelevant for comparison purposes or to assess his career, which pretty much imploded the day Argentina beat Italy in the semifinal of
their World Cup. There's a clear before and after.