There's this thing known as "progress". It's the reason why we're not sitting in caves wearing animal skins any more. Look back to the first MTA and then to 0.4.1. MTA has come on in leaps and bounds. The fact of the matter is that we know very little about 0.4.5 and so can't make any judgements as to potential usage patterns until we know more. I know that they've received several thousand bug reports from the error reporting tool and have spotted several patterns. I also know that some people were put off by the bugs in 0.4 / 0.4.1 and hence bug fixes alone may boost the fanbase.
They may also choose to combat some of the glitches that people use and address some of the criticisms aimed at 0.4.x.
Actually, in Tribes games, the most popular mode is CTF, not TDM. In Battlefield, you don't really get a choice as it's basically one mode, CS is basically variations on one theme, and in my experience, CTF is more popular in UT. I haven't played any of the other games.
What I'm saying is that GTA's gameplay has a lot of potential online, and it's not going to reach its potential if people just rip off game modes from other games.
So if it's "one hell of a fun ride", why can't it ever become "super popular"?