Remember, the vast majority of people who pick this game up, will do it because they like rugby and want to play rugby (albeit in an electronic format)... automatically they are going to be enclined to not quit for quitting's sake.
This is not some big, brash, yearly release, money churning monster of a game that every douchebag on the street is gonna want to pick up because they got the last one and because all their douchebag mates have it too... its more of a niche ***le that by and large the people getting it will have a real interest in picking up (this is more true in the long term, particulalry after the "just here for the RWC" crowds diminish). I think primarily, rugby (both the sport itself and the controls for it in game format) is too complex for a randomer to invest in without already having a good idea of what its all about... and its this fore knowledge and interest in the game that I hope lends itself to less quitting.
Sure I hope the game sells like a money churning monster of a game, but if it does I then hope the shite-hawk generation who are prone to quitting all **** off quickly afterwards - we want your money, not your company.
If there are less players online, chances are those players are going to be keen enough to play through a full game and not quit as it won't be just as "throw-away and jump in the next deathmatch game available".
Perhaps I've got it all wrong, but comparing a sports ***le to a FPS in terms of reason for playing does not seem a logical thing to do.
I play Fifa, I play it quite a bit, am quite good at it IMO (thats based on not remembering ever being well beaten or thrashed, my games are close, but I don't always win... just mostly
), but I play to enjoy a game of football against another person who brings their own style of play to the table and I play it as I would want to see it played on TV or at a real match. I honestly do not know what my win/loss ratio is or what level I am at and I don't care. Christ, I almost enjoy losing as long as I play well.
Added to all this, the fact that you are already on TRF, means you will have ample opponents to pick from without the worry (mostly) of rage quitting occurring (or whatever its called).