Yeah I just think it's unsustainable to have 5 top quality CBs in one squad. We've seen Laporte wants a move, if we buy Gvardiol or someone to replace him (or a proper LB and Aké goes back into CB), then someone else will probably want oto leave this time next year and we'll have a little rotating door, which is totally OK.
I don't think it's unsustainable if 4 of them are playing every game, which is pretty much what's happening. Though maybe under this system we need to class Walker as a centre-back too, but even then 6 with 4 playing isn't excessive.
Your right about the flow of players though, it's not a problem as long as we only have to replace one defender per season or something like that.