It's funny in a way. Barca's numerous meltdowns over the years have really narrowed down who the culprits can be on the pitch, we have a huge sample size to look at. It all started with Bayern and ended with Bayern.
Bayern beat us 7-0 and then after the following Tata season, Xavi stepped down from starter. We still got destroyed in the years after. Alves left after Atletico, still got destroyed. Mascherano after Juve. Iniesta after Roma. Rakitic benched after Liverpool. Each time we still got destroyed afterwards.
This doesn't absolve those specific players of blame at all, but there's gotta be some common denominator throughout all of them. Someone who has been there for it all. That leaves Messi, Busquets, Pique, and Alba.
Each one of them as well has their own major faults in our defeats. Pique/Alba commit numerous errors defensively and have a mental breakdown. Messi doesn't give a fuck when the ship is sinking except for maybe the Anfield game.
But the real glaring pattern in each one of those games is how badly we got pressured and how badly we reacted to it. Honestly I see Busquets and his playstyle/weaknesses as the consistently major problem for us. He has always been extremely strong in some areas and extremely weak in others. Particularly in the CL, he is just not suited to those end to end type games. When the conditions are fair-weather, he plays great and doesn't get exposed. But overcoming significant resistance becomes a struggle for Busquets. He has never been great in away games and even under Pep era we were way weaker in away games than at Camp Nou. Busi always benefits from home game and bigger pitch, his performances are really strongly correlated with that.
Even as a press resistant player, his game is just not good for the team under pressure unless we have a Xavi or Iniesta or Alves to do the off ball running so that we can circulate and push back the press or even counter it. He just doesn't move off the ball much at all. He doesn't follow up on his passes consistently. 1 touch pass out the back when your whole team is getting swarmed is something you can only really get away with when the squad is at a really high technical level like Pep's Barca or 14/15 Barca.
Busquets just isn't conducive to modern football and it is clear how horrible he performs and the whole midfield becomes when there isn't a world class mid next to him. When Xavi declined and we pushed up Iniesta to accommodate Cesc, Busquets became crap. When Iniesta declined after 15/16, Busquets became crap. If he isn't supported by world class midfielders he becomes way worse than usual. So yes, he WAS being covered for by Xavi and Iniesta.
The meme of him being underrated got so out of hand that he became vastly overrated and people forgot his rather obvious shortcomings because of some great strengths. Makelele 2.0 where the "silent engine" meme got spread so much it became the truth even though it's just not really that true. Busquets has been able to go his whole career with glaring flaws like his away game performances being completely unnoticed by the fans or media or anyone.