Even if he is «past it», lets say at 50-75% of his prime, he still brings experience and physical attributes that makes him , at worst, on par with Lindelof, which means we can rotate more effectivly.
I mean, would anyone chose Lindelof over Varane in Real Madrids side? Hell, Maguire probably wouldnt get in there.
Also, another reason he is suited for us is that he had played much of his career alongside a somewhat slow but physical and «leader» of the the defense. So he would take less time to beeing used to playing with Maguire.
One could look at Thiago Silva , who is much more «past it» , but has done well in Chelsea