Sounds like a crazy concept but I think it wouldn't be the worst for Barcelona to sell Messi. He's still carrying them but his role in the team is so overwhelming that I think players are willing to piggyback on his play and don't take the initiative themselves. Without Messi, Griezmann could play where he wants to, they would have enough to get Neymar in probably and it would last more than a few years.
Messi is unreal offensively but can no longer track back. With the way full-backs bomb forward now, there's no cover if he plays on the wing and you have to fit the team around him. He's 33 years old now so that part of his game is only going to get worse. He gets paid an absolute fortune a week. As of now, he's still worth it as the greatest player in Barcelona's history by far. But when he's 35 or 36, will he be worth more than a million quid a week?