People always underestimate the german youth. Kimmich was mostly a nobody before he turned 20 years. Lahm wasn't even a starter for the german youth NTs back in Germany's dark age, Müller was wasn't a "star" in the youth, Gnabry was highly rated but then took a nosedive, Sane wasn't even nominated by german youth NTs until he got playing time in the Bundesliga, Reus was never nominated for any german youth NTs except for just 2(!) u21 games and there are so, so many more examples (see also Klose, Schweinsteiger etc.).
In german football players like Kroos (very early on recognised as mega talent and then just went on to become a world class player) are the exception while most of our "stars" didn't look like exceptional youth players.
Having said that the current u18 and earlier age groups are again very strong and with Rhein there is even a CM talent I'd rate similar to the young Kroos, not to mention Moukoko (who is probably as close to a Mbappe level talent as you can get).
Still I'm sure there are a couple of players noone currently rates and will become great NT players.
Not everything is just about winning a trophy. Dortmund didn't exactly cruise into the CL last season and they need a player like Sancho to make deeper runs in the cup competition. Besides that if Sancho wants to win things that not an argument to force a transfer to United.
I agree and disagree with your ascertion.
I don't agree that Lahm and Schweinsteiger were unknowns at the age of 18-19. Lahm was already one of the best LB's in the BL by 20 after having a breakout season with Stuttgart, enough so that he started in Euro 2004 as a 20 year old, same for Schweinsteiger who was 19 then and already had 40 BL appearances. Both were regarded as top talents by that age. The reason why neither had significant U21 appearances was because they were already part of the senior team at 19-20.
Reus and Muller, I agree, were not highly regarded at the same age. Reus was considered a talent but not expected to be as good as he has become, it wasn't until he was 22-23 did he really show his true abilities.
If you look at the core of the World Cup winning squad, most of them having won the U-21 title in 2009, a lot of those players already had breakthrough seasons at the age of 19-20 and were stars of their sides. Khedira was a major part of the title winning Stuttgart team in 2007 (19 years old), Ozil had won the DFB Pokal in 2009 (20 years old), Kroos was one of the best performers in the 2009-10 season (19 years old), Hummels was a key player for Klopp's Dortmund at 20 years, Howedes for Schalke and Boateng for Hamburg.
You can't find a similar group of German players aged 19-20 in the BL right now who are playing such a major role for top 6 teams, Havertz is really the only one. There were plenty who have shown promise but stagnated a bit, Meier doesn't start anymore for Hertha, Geiger has suffered injury problems the last couple of seasons as well. Can't think of many more in the similar age group otherwise.
The part I can agree on is that the BL is the best place for youngsters to develop and therefore it can be that we will see players become better than their European counterparts due to getting a lot of playing time at ages 20 and above. That being said, I think the number of German youngsters at big clubs has fallen by quite a lot compared to the late 2000's. English and French talents seem to be on the radar of bigger clubs now, as evidenced by their signings.