What I notice watching the team is that they play with no fear and they rely on a lot of individual talent, directly taking players on etc. Senior football is far more tactically restrictive. Also the physical gap is reduced, if you see England's U20s against Italy's for example, or how England just kept running over teams in this tournament. I'm not trying to take away from their talent because there are some really good players especially in the U17 side (I'm not so convinced by the U20 side individually), but now you've got athleticism mixed with skill and a bit more brains, they try to play the right way.
Playing in the Premier League is also a physical task for English players, too many silly cup competitions and no break does wear them down. There will now be increased media expectation on this generation, and that never settles well with English players. They have a tournament fear and that needs one good tournament to be rid of.
Anyway if you look at the Brazil U20 side who did it in the same year as their U17s the best players were the likes of Dudu, Daniel Carvalho, Adriano, Nilmar, Dani Alves, Fernandinho. The U17 side had virtually no one of note go on to become senior internationals, Arouca and Ederson have 5 caps between them.