Here's the thing.
He is not bad, a football genius in the head infact. And he can probably go until 35 because of it because his stamina is very good, just his speed and strength are not, and never were.
Slightly different eras, but it is like when Milan let go of Pirlo because he was too old for the game, and then he goes on and gaves 4 more seasons with Juventus.
The problem (and it was the same with Messi) is that you can not have a core of Busquets, Pique, Lenglet and all these unathletic sacks and expect to be competitive. If we want Busquets to be the DMC maestro for a few more years then the whole team surrounding him must be fit, fast and at their physical peak.
If we wanted Messi was our one man attacking gang, then the same. And the same with Luis Suarez if we wanted to have an old striker who just scores goals. But we can't have them all together, so right now if we choose to maintain Busquets (which is the right decision imo) then all of the veterans or slow and unathletic players around him must go (so Sergi Roberto, Pique, Umtiti, Lenglet and maybe Eric in the defensive line).