Football is moving away from club support to player support. One can argue that player brands these days are bigger than club brands. I know it's a bit simplistic but just look at social media ranges. Messi has 220m Instagram followers, Barca is at 97m. At least in the short term, they will lose a ton of revenue if Messi leaves. Also, wasn't Barca miles away from the financially elite clubs before the Ronaldinho/Messi days?
Whether or not Messi is worth this salary for sure is debatable. I was of the opinion that he was based on the studies quoted by Laporta but
@Iker Quesadillas has brought up some profound arguments. So it's entirely possible that Barca could compensate for his loss and achieve similar revenue numbers a few years down the road but in the end, there are clauses in the contracts are very real and they'll definitely have to take a hit on their revenue short term wise. And in their current situation, this may be very hard to deal with. They'll lose the "Messi payments" negotiated in the sponsoring contracts, they may not lose 80% of shirt sales but maybe 20-30%, etc.