I think we just disagree over the definition of being short. Imo we need two players in every position, plus the EDS. This appears to be the strategy of the club too. In centre mid we have Foden, Gundogan, KDB, Bernardo. Injury risk exists for every player in the league but this represents better strength in depth than any other club. That's what it's there for.
Foden replaces David Silva as a member of the senior squad, Doyle replaces Foden as the young prospect. Doyle won't play a handful of cup games if you put yet another midfielder in his prime into his way. There has to be some space to develop young players and your approach would suffocate that. As I also pointed out, bringing in more and more players for the same position costs money: the transfer budget and wage budget have limits. Spreading those budgets across extra players we don't need means there's less to go around to retain top performers. You don't appear to be taking that into account either.
By all means critique the qualities of players we have but the answer can't be 'we need 3 players for every position'.