the problem with MW2's multiplayer is that it's broken, unbalanced, boring, and infested with camping. there are plenty more reasons, but these are the most pertinent.
assualt rifles are ridiculously godly (to the point that there no point in using any other gun), the maps are poorly designed and encourage camping, the spawns aren't any better, the game requires a lot more luck and almost no skill, and the multiplayer just plays like a giant game of hide and seek sometimes. any given TDM game in CoD4 will feel very fast paced, intense, and sometimes makes you feel like you are on a battlefield. whereas in MW2, you might struggle to find people unless you have a heartbeat sensor.
the great thing about CoD4 is that it is somewhat balanced, the maps are designed so you can play anyway you want (run and gun, camp, run and gun sniper, etc) and still do well. while certain weapons are better than others, it's still not hard to do well with a given class or weapon if you put in enough effort. it's not without it's flaws, but it's a hell of a lot better than MW2.
but it seems that those who never played CoD4, seem to love MW2. those who played the hell out of CoD4 seem to find MW2 disappointing.
dying community? the game is more than 2 years old, has a sequel out for it (that was one of the most anticipated in gaming), and it's still very easy to find plenty of games in any given mode. if that's dying, then i really don't know what to say.
and i bet you are one of those campers in MW2 who uses an AR with SP, SA, and a HBS. look, i can make dumb generalizations too.
and have you guys noticed a trend with most people who advocate MW2? whenever someone comes in and posts a somewhat tangible reason as to why he didn't like MW2, a MW2 fanboy will rush in and assume he hates it because they suck at the game. then they'll make vague and stupid generalizations, and not give any good reasons of their own.
i had a K/D ratio of 1.98 before i stopped caring and went crazy with a riot shield and tried running and gunning with an RPD. i could care less about MW2's mp. and i sucked when i first played CoD4, but i loved it despite sucking.