The whole 8 out of 10 shirts have Messi's name on is a misnomer IMO. If he left Barcelona then I imagine fans buying the shirt would just get one with the next best player's name on.
Possibly. But would as many people travel to barca to watch a game if messi isn't playing? You've seen how many koreans come to spurs games just to watch son. Each buys some merch in the store. Then you have sponsors, they wont pay barca as much if they don't have possibly the greatest player who ever lived playing for them. Also without ronaldo and messi will people pay as much for tv rights for la liga? Lastly what he brings on the pitch. A large part of barcas success has been due to messi in the last few years. Messi leaving this summer would hurt barca a lot and there is nobody i can see they can replace him with, especially with the financial difficulties they have.