That's kind of my point, I wouldn't be looking to buy another midfielder if we were going with the 4 of the Bernie/Kev/Dave/Foden as that's more than enough. Maybe a young player to add depth (hell, Zinchenko would probably be front of the queue), but in that scenario, someone like Gundogan is excessive and frankly he'd be worth more to someone else than to us as a 5th choice who we have to keep finding excuses to play. I only mentioned the possibility of signing someone as an option if Dave left or something happened to make that 4 unviable.
At the moment, we have 3 players almost equal for those 2 positions, then Gundogan who is levels below them as 4th choice, then Foden who is probably still levels below him as 5th choice but with potential to be better, but in need of games to get there. That's fine for now, but as time goes on Foden needs to overtake him. We can't just let him stagnate as 5th choice until Dave leaves
Well, first I don't see Gundogan as 'levels below', he's a top class player who's lacking a bit of dynamism (compared to Kev and Bernardo) thanks to repeated injuries. At Silva's age that's a problem for him too and one that will get worse and imo very soon Gundogan will be clear 3rd choice whether Silva is still here or not.
That's when we will get a better idea of Foden's ceiling, if he can't overtake Silva and challenge Gundogan over the next 1 to 3 seasons he doesn't have a future here tbh. That might sound harsh but it's true, any player that isn't at a level to challenge for a first team squad spot on merit by 20/21 should leave.
If we sign someone else significantly better than Gundogan, ie KDB level, Foden doesn't have a hope and should leave. Bernardo and KDB haven't even hit their prime yet and the lad can't afford to wait around until he's 25 and KDB gets to where Silva is now. Lets face it, if Silva was 28 years old still Foden wouldn't have a future here. That's what you're asking for. 3 top class players in that position means he'll only be assured of making the bench when one of them is injured. That's stagnating.
Then there's squad balance, anyone significantly better than Gundogan will have their pick of top European clubs, as would Bernardo and Kev if they were unhappy and if we have 3 players in their 20s at that level for 2 positions then one will certainly look to leave. World class players expect to be key players for their clubs.
Then, we'll end up signing someone who's either at a good age and happy to come in when needed knowing they can't play every game (Silva), someone who's talented but a raw prospect (Foden) or someone who's a quality player in their prime but not quite world class who'll happily fight for starts from a squad spot (Gundogan).
You can't have 2 world class players in every position as world class players want to play, need to play to stay at that level and have top clubs fighting for their signature.
Hence we have 3 top class wingers at good ages for 2 positions with Diaz gone, 4 fullbacks including 2 utility players (tried to get rid of Zinchenko), an unhappy 2nd choice international keeper (before his injury), a mainstay backed up by a talented but raw prospect a decade younger at striker and we only get away with the depth we have at centre half and in centre midfield because of Vinny's age and fitness issues, Silva's age and Gundogan's injury problems.