I am not sure what you mean by Walker was never the best defender.
Maldini Company and a few others may have been better but I think he has turned himself into an outstanding defender. When he is playing there are very few threatening attacks coming down the right, he is excellent 1v1 He clears a lot of balls from the other side. Defensively I think he is now outstanding what he is less good at is quick use of the ball and productive use of the ball when he uses is speed on overlaps.
Coman was giving us real problems when Walker wasn't playing ditto Vinicius last year in the semis.
My point is that he is an overall great defender because of his immense athleticism and pace. He has never possessed inherent exceptional defensive ability (like Ake, Dias, Kompany, or Maldini, as you mention).
He, more then anyone, understands that once his athleticism and pace begins to decline he is going to drop levels very quickly (as opposed to the aforementioned players, who never relied on their athleticism or pace to defend properly).
He has been able to pocket some of the best wingers in the world almost entirely because his pace and power allows him to match them step for step or, if they outwit him momentarily (which has happened a lot), he can recover and reengage.
And, even then, that was only really after years of being coached by Pep and co, learning how to position himself and shape his body to allow him to recover and keep up. He’s still not especially great 1v1 or positionally.
Once more, he was going to get less playing time next season because his ball control, passing, press resistance are not good enough for Pep. Hence why Rico was preferred for awhile even after Kyle was back from injury. And why Stones is often preferred when we aren’t up against pace, but rather technical ability. Pep literally said this several times himself.
Will he be missed? Definitely.
Will we miss him more then we missed Silva, Dinho, or Sergio? I don’t think so.
Are we going to be worse off? No.
I’d be happy to make a similar bet with
@domalino that I did about Stones eventually becoming as good (if not better) than Kompany (at a time he was calling me an idiot and a clown for making that claim).
Again, the only irreplaceable member of our team is Pep.