Who did Ronaldinho play with at PSG before joining Barcelona ?
Or Hazard when he was at Lille ?
Or Ribery when he was at Marseille
Or Kante when he was playing in France 2nd division ?
Or Ruben Neves who was playing in the Championship ? Or Maddison ....etc
What about looking as Sancho and see how he preferred playing regularly with less talented group than staying on the bench with exceptional manager and players ?
Sancho left because he wasn't making the bench btw.
Not quite sure of your point in naming all those players who joined other clubs, they weren't loans but transfers.
They left a smaller team for a bigger team with classier players and improved, which more proves my theory than your own for Foden going on loan or staying and playing with our own world class players.