It's not that there aren't enough people willing to pay, p2p MMOs existed for many years, even WoW has been p2p for almost 9 years already and it doesn't looks like Kotickvision Blizzard is having financial issues over that.
What happened is that the definition of "what's enough" has bloated, f2p generates more cash than p2p because more money is spent on average. In order to unlock all of the normal gameplay features, people have to pay way more than what a standard monthly subscription would cost and it usually applies separately to all characters. Toss in winged unicorns and kawaii bunny suits (stuff like this is completely out of place) for retards and you're set.
It isn't as bad when the game has some sort of "gold subscription" plan which is basically your normal monthly fee, unlocking all the features (even though it tends to cost more) but even then the segregation effect and the general feeling of dumbfuckery is present, cash shops feels inherently stupid and disrupting.
f2p also attracts undesirable masses of all kinds, kids, feral foreigners, farmers (that always undermine the economy, there's no way around it other than not having an actual economy, kinda like WoW does), sperglords that join and quit all the time (hurting guilds and such), etc. People that choose to pay a monthly fee/bought the game do it because they are determined to play the game after having played their trial/first free month.
The actual balance related issues of f2p model are minor when compared to what it does to the game and community iself. Not even talking about the aberrations that are asian f2p MMOs.
In my eyes, "f2p" should work like an unlimited trial of sorts but with a time limit, after which people can pay the monthly fee and upgrade their account, hell, the fee can be made weekly, daily or even based on actual time spent in-game, using a fair pricing system. The most discouraging aspect of p2p seems to be "wasting days", not being able to play often enough to justify the cost. With such a system the worries would be eliminated.
Though, I actually prefer the classical monthly fee since it makes the players more commited.