Football is very much about today. It shouldn’t be, but it is.
Birmingham City had a kid who was good enough to play, so they threw him in. Had Birmingham City been guaranteed to keep him for his whole career, they could have used him more sparingly, and introduced him slower. But he was good enough, so as a 16 year old he played 41 league games, helped them on the pitch, and then got a large (for them, and for a 17 year old) payday from Dortmund.
Dortmund signed a very good player, but presumably they knew they would not have him for the rest of his career either. Their model is to give top young talent the experience and then sell on for a very big fee. So they played Bellingham, and he has definitely been good enough to have them fighting hard for the title, as well as holding their own in the Champions League.
For Dortmund, their objective is presumably to enjoy the player for 3-4 years and then sell on for a very big fee.
Bottom line, yes, it is a concern how many games Bellingham has played already, but football is very short term, and everyone is concerned with their own interests, so what has happened so far is understandable.
What remains to be seen is how many games Bellingham will play at a high level over his career. 500? 700? 800? More?
If we buy him, hopefully we will have a good enough group that we can rotate him from time to time, to get some rest into him. But even at that point, every governing body in the game is adding more games to their competitions, so protecting a player, for the club, is an increasingly impossible task.
It needs someone to have a career long view, and it needs the various bodies, domestic and international, to make a concerted effort to reduce the games the top players play.
Good luck with that!