He's not.
What makes you say that?
For me most the time I hear people say that it’s just nostalgia and sentimentality. You could have God playing in front of them and they’d still say he’s not the best ever, because no one can top the players of the past.
For me it’s not even that close. And it makes sense, because with time you’d expect players to improve. Specifically Beckenbauer and Baresi though, for me almost midfielders, with the goals they scored, talent with the ball etc.
But as an all-round central defender, his physicality, pace, reading of the game, tackling, timing, calming, play at feet, the list just goes on. He does things on the field I’ve never seen a defender do. And not spectacular like a Ramos, but he makes the simple things look incredibly easy, as if they were always meant to be, like a Beethoven symphony. Watch him make a tackle and it’s a piece of art. That highlight reel of him from this season alone, find one of another player doing things even close. I don’t think you will.
But yes, he does need to sustain it further to cement it.