There are different types of users - the big token buyers, the medium ones, the small ones and the ones that do not make any money purchase, but what most of those users play this game for? Mainly for fun and entertainment, except those few 'farmers' that do blackmarket business by selling accounts.
The main reasons for quitting are expected be the same. It is when the game is no longer enjoyable on overall - when the negatives outweighs the positives. It might be also a matter principles as some users after having seen the scammy sides of the company might opt to stop because it is against their principles to support a company behind such acts. Then comes to how much can a user play (and achieve) with the resources he can have from the game itself, which comes to the what like some users previously said in this thread they will quit if they cannot play this game for free. I think this is like an indirect blackmailing message.
In fact, no one has played this game for free as such. There is still the cost of internet, the cost of the device and the effort that has to be put in for the offers and so on.
Briefly, a user will quit when on overall it is no longer worth it for him to continue to play.