You guys are losing a bit of business sense here. Why does it seem that everyone is making FPS games right now? Because they sell really well. Does that stop them or other companies from making other games? No. You are making the assumption that if every "party" game sells well, then every company will switch to making only "party" games. But not every party game even sells all that well. EA Playground didn't do too well, and I think I remember it being advertised. It even has dodgeball, which is almost a sure buy from me. I love dodgeball. But what happens when all these companies start making only party games? Where does that leave people who don't like party games?
Well you're in luck. Because once a company looks around and sees an oversaturation of party games, they'll realize there is a demand for the typical "hardcore" type game, make one, and with millions of customers starving for a true hardcore game, it will sell 20 million first month. All the other companies seeing this startling great selling game will shift resources to making hardcore games.
That's what would happen if companies were run by lesser men and who were very short sighted. But since there's demand for all types of games, there's no need to completely stop making those types of games. And since the party genre market is growing, that means the industry is growing, which is a good thing. And as more companies learn what a good party game is, and more competition between companies over party games, you'll see the quality of party games improve. Though I don't think reviews will reflect that, because reviewers typically don't like those games, so they'll still not get good scores.
This basically happens with consoles. One console is decided to do well, and thus is given preference for resources. If another console starts doing better, more resources are moved to it, to take advantage of the growing userbase. It's all about supply and demand. As long as there is demand, there will be supply.
And don't worry about lack of quality games. The gaming industry crashed because too many games sucked, so people stopped buying them. Now with more quality control, more competition, more awareness (through reviews), it's harder to continually put out crap without a backlash. Think of Sega. Seems like everytime a game is announced people write it off and say "well let's hope. but it is Sega." I assure you, we'll see more party games (of higher quality), but we won't see less of other types of games.