It's true Busi is getting older but watch his performances with Spain especially when he's paired up with Thiago, He looked much better than he did for us. It wasn't just the system that helped but he also had a CM in front of him who not only had the legs but also knew what to do with the ball and can can withstand being pressed.
Busi at Barca had to play with an aging Iniesta who didn't have the legs he once had. Rakitic who while good in his own right had a number of limitations and he had guys like Gomes, Paulinho etc who are either not good enough or so widely different in terms of style to what the team is used to.
It's no coincidence that Busi played his best Barca match in a while with Arthur starting right in front of him. Surround Busi with players who are technically competent and you will still get some of that vintage Busi.
Busquets is still very good on the ball, last season was one of his best with the ball
It is defensive duty that is affected, and sure a different system can hide it, and in Spain they play double pivot a lot and it usually helps, last year it was covered most of the time when he and Raki playing double pivot in 442 and Paulinho had the lungs for it but even a tough team like Chelsea exposed him badly (Willian repeated shots was his mistakes) although those games Paulinho was in bad form and didn't play second game.
This year with 433 he is exposed badly, I don't think it is just "the Arthur effect" as some people portraying it and has to do more with that extra midfielder we had.
That is why I think the Busquets- Athur- Coutinho midfield that people wants is a disaster waiting to happen, it will expose our midfield badly