That can be different for everyone but generally when multiple muscular injuries start it's more difficult to regain full fitness for a player of his, Vinny's or Micah's size and stature at any age simply because of the additional strain on their bodies. Koulibaly has had multiple niggly injuries recently and at his age, it's only going to get progressively worse in my opinion.“From now on” suggests that there’s a point in time when fitness becomes a concern. Kompany had injuries throughout his time with us and Richards is 32 now and also had injuries throughout his 20’s.
As for breaking defensive teams down, we haven’t had a problem in the last 3 seasons so don’t see a left footed centre back as integral to our attacking.
And anyway, we’d have the choice of Laporte or Koulibaly against every “smaller” team.
I'd say we've struggle this season without Laporte, both defensively and offensively. Pep has said multiple times he prefers balance in his CB pairing because it speeds up build-up play and enables us to transition the ball from one side to the other quicker. It's not absolutely necessary admittedly but it certainly helps.