Well is Cristiano Ronaldo one of these great ones or is he not? Because, by my estimation, he has broken the mould and become the first person to muscle into the 'GOAT discussion' without the requisite ability. My contention is that if he can do it, others can do it. I think the parameters have changed, not sure why you think they are set in stone.
The first thing to point out about C. Ronaldo is that, whilst he didn't have the ability of those in that S tier, he was still an exceptional and rounded/complete player in many ways/facets and that is slowly but surely being eroded when he is discussed, like, his ability itself is being reduced to bring others into discussions particularly with him.
The current batch of players contesting for best on the planet are collectively a weak bunch in terms of technical brilliance and
stupendous ability, meanwhile, C.Ronaldo always had crossover with many who are regarded as very special when it comes to technique, so he was being judged to a higher level of acumen out of the gate - his direct peer/rival was also the only player people directly compare to Maradona 1:1 in terms of outright ability. It's not some random bar or erratic one, it's the highest there is - earlier in C.Ronaldo's career, players like Ronaldinho and Zidane were still the reference point, and next to all of them, he technically cannot outdo the collective. Remove all those guys from the landscape, as Mbappe has had the privilege afforded, and C.Ronaldo is perceived very differently to how he was.
Mbappe holds a candle to none of them, to a greater degree, probably, due to his reduced array of skills, and it's not collectively talked about anymore because it doesn't really matter until all-time scrutiny rears its head, like it has in this discussion. What Mbappe is is sufficient amongst his peers, and that's all that matters, until we're no longer talking about his peers. It's not just Messi and C.Ronaldo and their scoring escapades skewing the landscape; it's a previous generation of outstanding technical footballers: Ronaldo, Zidane, Ronaldinho, Messi, Neymar and so on and so forth. Currently there's no successor to any of them, not even close, but the moment they are mentioned, they make dwarves of what we currently refer to as the best of the best. So when you compare Mbappe to C.Ronaldo, it is not a like for like, and despite C.Ronaldo not being able to take over and dominate games like those he is scrutinised against, he could do so to a considerably greater degree than we've seen from Mbappe in club football.
I understand what you mean by C.Ronaldo muscling in via sheer determination and consistency, but what C.Ronaldo is, Mbappe hasn't shown to be anything like as of now, so it isn't 'well why not Mbappe too?' in my head because it has no grounds - Mbappe hasn't shown enough of himself being a juggernaut in the club game to be comparable to the Portuguese or even to follow his path. By the age Mbappe is now, C.Ronaldo was going into hyperdrive and taking the club game by storm. Can we say the same of Mbappe? Not really.
These two are at odds with the normal scaling, also - Mbappe has a phenomenal international pedigree, where C.Ronaldo did and does not, but 180'd, C.Ronaldo had a club pedigree that invokes names such as Di Stefano and Puskas, where Mbappe definitely does not, so they are two incomplete players in this way needing the other ones 'half' to make a whole in the way that's usually needed for literal greatest of all time discussion.
Mbappe will have to take club football by storm with his transfer to move the dial. He's been stagnant for quite some time now, so that's an interesting proposition in itself. Can he go to Madrid and tear up trees and become a phenomenon in his own right? And to what extent? In these hypotheticals where he conquers the world, can he suddenly become a player that the game revolves around? I may have sounded harsh in saying it's impossible for him to reach the top table, but imo, he'll have to do something we've never seen before (and what I believe he is incapable of) in controlling and outperforming all to such a degree, it wouldn't even be associative of the Mbappe we've seen throughout his career to be possible. It's not like a young Maradona achieving true ascendancy at 25, or Messi realising his potential, or Ronaldo just carrying on his trajectory for a period of time - it's a player who has not shown that level suddenly becoming something wholly different to what he has been his whole career. Performance is what moves the dial, far more than goals in and of themselves. "Performance for the ages" is what sticks, not goal #455 or goal #456.
And the parameters never change, no. They remain steadfast and resolute, which was evidenced throughout the inanity of the Messi vs. C.Ronaldo borefest. In the end, once the dust settles, everything stays how it has always been when it comes to final evaluation, which is precisely why someone like Maradona has not and will not be erased from the timeline, despite not fitting this modern criterium in the slightest.