Very interesting point that you make. I just wanted to ask you, its the system which German Clubs use which make players perform better than they can individually, because I get the same feeling that Sancho will be very similar when he leaves Bundesliga.
German teams in general are very focused on collective compared to their English counterparts. This reflects the image many Germans have about how to play football ("11 Freunde müsst ihr sein") but also is a tribute to the financial disadvantages German clubs have (always had) compared to their international competitors from England, Spain and Italy. I feel like there's much more emphasis on playing style, system etc. than it is in English football where the focus is more on individual quality.
I think that's part of the reason why some players who did exceptionally well in Germany couldn't replicate that abroad. Naturally a functioning collective enables you to perform much better than you would in a less well drilled team and if a player is used to playing in such a unit, it's harder for him to adapt to a more "improvised" tactical environment. That for example happened to Kagawa and Mkhitaryan, I believe. But I'd say the same would happen to players like Mane, Salah or Sterling, currently.
Now the question is, which player is better - the specialist who flourishes in the right environment and hits a very high ceiling there or the generalist who does well in every environment but might never reach those heights? Since IMO a top club should always try to implement a system, I'd personally go for the former and try to build a team of players flourishing in similar environments. That's essentially what Klopp and Pep are doing.