What about this season, though?
Neymar is irreplaceable, he's the most marketable player on the planet right now, if we lose him we lose a lot of our revenues and future fans.
Little chance of him leaving this summer. Little chance of Barca getting Verratti or Dembele now too.
He's not irreplaceable. Mbappe will replace Cristiano as Nike's next cover star if he kicks on, just like Pogba will replace Messi when people thought it would be Bale or James.
Neymar's hype have dropped off last few years and if he keep acting like an immature kid all the time, he'll never play to the level we all think he can and should play at.
Fans can leave if they all follow the club just for Neymar.
If the club can get two of the most talented players in the World in the last decade(assuming they take big steps next season both two),
You
1. Secure a large part of the club's future
2. Solve the midfield problem if you get Verratti in exchange
3. Have the means to challenge more and better for trophies which will eventually gain Barca those fans and bigger deals, more money
The problem obviously would be the club's incompetence in securing them or even Dembele and a guy like Hazard or Dybala.
But there's a plan the club can follow if Neymar's dad want to take his son to Paris next year. PSG cannot pay the release clause, but if Neymar want to leave next summer, the club won't force him to stay and should not be held ransom by the Neymars.