First part is true. In my opinion we don't actually need Aurier, not right know anyway. The situation with Aurier is slightly different though, compared to Fabinho.
He's already a sub at his club, rather than a starter, and out of favour. The prospect of being a sub for a year and then taking over from Valencia would be an improvement on his current fortunes. Such would not be the case for Fabinho.
Very crucially though, we know Aurier is a RB and happy to be that. We don't know that is the case for Fabinho. If approached to be signed mainly as a RB, Fabinho might not be interested. And if he did come, he might well end up being not as good a RB as Aurier on top of being more expensive due to his elevated profile from playing DM. Many unknown variables in this move.
So I guess I disagree that it makes more sense to splash the alleged 50m for Fabinho due to his versatility if Aurier is available for half that (I'm purely going off the mentioned prices here) and it's a RB rather than a DM that we want to sign.