Success in an MMO is whether there is enough money coming in to pay fixed costs + repay investors and get themselves out of the red.
So how successful an MMO is really depends on the fixed costs of said company vs. how much money that game is making...in so long as the game is making 1 dollar more than the fixed costs to run the game then it is successfully running. Now $1 is clearly not enough profit for anyone to keep it going, but profit is also a subjective thing. EA expects big returns on their investments and that is why the referred to SWTOR as a "failure" with 750,000k subs back during the last call.
SoE, Turbine, Trion and other MMO companies don't seem to have as high expectations (That and their games didn't cost as much to make as SWTOR) so they are perfectly happy to run their games with only 100, 200, 300 thousand subs.
I remember reading an article (though its be a while back) with an MMO lead dev who was saying that people would be shocked at the actual overhead of running an MMO. When you take staff out of the equation it costs companies virtually nothing to continue to run servers (I believe the article said an MMO could easily be run on less than 10,000 subs) and that those costs were going down due to cheaper technology coming out and this was many years ago.
The real question is how many devs are on a project and what their salary cost is.
At the end of the day it doesn't matter. EA will never shut this game down because they paid a ton of money to have it made. They will just keep cutting staff until this game is running on just a handful (less than 10) devs and will leave it to just generate any profit it can so it can recoup as much loss as possible.
actually Bioware Austin has been hiring people now that it went F2P. So it doesn't look like they are cutting more jobs anytime soon. Seems they are actually using the profit they got from F2P to put back into the game.