I think that sums up exactly where people are getting confused. Looking at players aged 28-30 and thinking they are "always" injured, so this is totally normal.
According to transfermarkt, since Ake arrived, he has been available for 80% of games we've played.
In the same time period, Stones has been available for 85%.
Dias for 90%.
Akanji in his two and a half seasons, 93%.
Kovacic for 90%.
So, while none of them are up there with Rodri (who has an insanely good career record, which is one reason why we can't simply get a like for like backup), it is highly unusual for them all to have problems at the same time. Pep said at the start of this run that Walker hadn't been training and shouldn't have been playing, and even Gvardiol was a doubt at one point in the run, but had to play.
ps. The point with Walker is that at 34 he was supposed to be playing a final season with limited minutes. When he's not got recovery time, not got a fit and fast CB beside him, and Rico is playing left back, he's going to look like he's deteriorated a lot faster than he likely has in reality.