F2P had an influence sure but i cant see it being as much as bad balance, oppressive heroes or matchmaking.
Yes, quitting is a state of mind thing. If the game puts players in that state of mind, then they’re more likely to quit. You can either punish players for reacting to what the game did to them, or you can stop it doing it to them.
Fun is indeed an interesting term - its a core driver for any game. As such, it should be taken seriously. More fun for most people is good. The whole “my fun is more important than your fun” is silly and why Widow exists.
Denied nanos caused quitting in OW1, when quitting wasn’t such an epidemic.