All reports state the main issue for the re-signing was fitting Messi's money into the new salary limits imposed by LaLiga. The proposition made by Barcelona's management was for Leo to earn 20M € this coming season, to help accommodate the new guys and ease the economic situation the club is undergoing. This would be rewarded by the club, granting him a salary raise up to 80M € for the following 22/23 season, totalling around 100M € for 2 seasons.
The new salary cap limitations made this deal impossible, and management asked Messi's father to lower a bit further the salary in the agreement. And this is where his father drew the line and said 'No.'
So, recapping and making it simple: dude left for money. He is the best, biggest, dearest and most defining and important player this club has ever had. He has stablished records and numbers no one will come close to in the next 80 years. He has worn that shirt for 20+ years. And he is leaving for money. A dude who has earned well over 1,000,000,000 euros during his professional career has decided not to push these numbers higher, not to keep fighting for the team of his entire life, for money.
I am certainly not able to speak from the perspective of a billionaire, so maybe I'm not suitable to decide if it is worth or not, but, from the outside, it definitely looks like going from 1.3B € to 1.6B € in total career earnings when it's all said and done is not worth leaving your lifelong team and abandoning the chance to stablish numbers that won't ever be matched.
Of course, Barcelona's management has been GODLY AWFUL for the last 5 years or so. But he's not playing for the management, he's playing for the team, for the fans, for his supporters. It'd be a totally different story if he came out and said 'I think this team is not prepared to compete for the greatest challenges, I consider I deserve to take a look at other options'. I would respect that decision 100%, because the will to win would be the fuel for it. However, he was ready and willing to sign before the last salary adjustment was proposed, so that wasn't his main concern.
It's just money. It's just having a 10% more wealth when he retires. LIKE WTH MAN

Can you imagine Dirk in 2016 saying 'Nah, screw it, i'm playing for the Hornets for 25M $ next season'. Or Duncan after 2014. Hell, it's worse than that, it's one of the greatests stars in SPORTS HISTORY. Top 3 ever in his profession at least. I dunno, maybe at the level of Jordan as a sports icon?
It's different from cases like Raúl, or Casillas, where they felt they could still play at a great level, but the club didn't so they left looking for minutes and chances. It's different from Ramos, because he didn't grow up playing for Real Madrid youth team, and he got to Madrid as a 19 year old.
Here, Leo, take 400,000,000 € from this random team instead of 250,000,000 € from your lifelong club, where you could retire as a legend. He's also leaving without a chance for the supporters to say goodbye on the pitch.
I feel like I could go on for hours.