Actually before coming to Chelsea is form already declined and he did better for us than Manchester United. His stat in the loan spell at Borrusia dortmund wasn't that good compared to his past stat the club. Also, the Bundesliga is less tasking than the EPL, how many players can you think of that couldn't perform well at the Bundesliga but did well in the EPL? But in the other way round there are quite some examples.
Don't speak as if Chelsea was the reason he declined whereas he was having a better performance there, trying to get back on track.
Sancho isn't a serious player, he was asked to reduce his wages to get a permanently moved to Chelsea but he refused, for a player that isn't playing like a superstar the way he sees himself, I don't understand why he doesn't want to reduce his wages. Chelsea could have been a good fit for him but since he didn't reduce his wages then he should go to the club that'll be willing to offer him that big salary without being worth it. Chelsea didn't decline him but he was already a declining player that has too much ego thinking he's on some kind of superstar level. When players aren't ready to reason with clubs that want them then they don't deserve to be playing for that club.