Success is somewhat a subjective standard in the mmo industry. There a some who claim that if you don't beat WoW your game is a failure. That's a pathetic metric from pathetic people.
Then there are games like EQ still chugging along, making money, and with a bare bones team of devs. I haven't played EQ in almost a decade, but the fact that they are still here and engaging players I would consider a success, even I'm not interested in playing it in it's current form.
Then there are games like New World, or City of Heroes, that made money and were popular, but they didn't make ENOUGH money for their corporate overlords and were cancelled. Were they a success? or failure? Depends on your viewpoint.
If M&M keeps the team size manageable while not overextending itself with frivolous expenses and feature creep, then they can be finacially successful even it the player count is a tiny fraction of WoW's. Of course most of the posters here and elsewhere will scream it failed even if M&M is still around 20 years from now. That's just the way most people are. They claim everything is a failure.