Which is more profitable – an MMO with a monthly subscription or a free-to-play game?

there's really nothing else to buy.

That's not true. For example, in PvP, there are some really cool items that cost approximately 1 million rubles in real money. Not to mention a whole bunch of other items.

there are relatively few such crazy people

Half of the top players.

Added 4 minutes later
And look, in an MMO with a monthly subscription, each player pays $10-15 per month, and in free-to-play games, almost every player who has a job pays at least 1,000 rubles. That's also quite stable.