This is what I don't agree with. Busquets was never at any point the focal option of the midfield like Xavi was or like Rodri has been for City these last 2 seasons. Even for the NT during the last Euros for example, Pedri was the focal point of the midfield, not Busquets.
After Xavi, Messi became the full time playmaker of that Barca team until he left.
It's not just about replacing him with anybody is it? We're talking about replacing him with him with any player near the same quality, otherwise any player can be numerically replaced on any team (they all have to put 11 players on the pitch after all).
And the same argument you're using can apply to Busquets. If Barca needed to replace him earlier, i'm sure they would have found a way to also.
I'll ask this though, other than being a bit better at keeping possession (which Rodri is also good at by the way, but I admit Busquets was slightly better at this), what other qualities did Busquets possess that Rodri doesn't? And what other aspect was he better?