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?
Barcelona need to re-sign Neymar and go hell for leather for the CL next year with Messi there. Their recruitment has been disastrous the past few years, culminating in Griezmann. I totally agree that he doesn't track back, and can't be what he was, but he has 2 more years of being a decider at the very highest level.
Barcelona haven't really changed their system recently to accommodate him better, and he is finding the great players around him either stripped away or getting older and slower. Yet his numbers remain astounding. I also wonder how much he's worth to them in shirt sales and publicity too, compared with other players. That must influence contract negotiations.
At a tangent, I actually think Pjanic will be a good signing for them if they're trying to win one more CL - he's a very very good player. But they need Neymar back, that whole thing when he went to PSG was terrible for Neymar and for Barca. And difficult decisions around Busquets and Suarez also need to be made.