I think that they aren't. In fact I think that it is a great thing that gaming is becoming more main stream. Makes for more money invested and that is never a bad thing ;)
I'd have to agree on that as well. I guess you could say it's fundamental, seeing as how the more they play, the more main stream it becomes as you said, which is how I would say it is. And another thing is the more comfortable gaming becomes, the more money gaming industries make which in the long run progresses the gaming industry.
I believe casual games are more like stepping stones into the world of gaming. It always starts by falling in love with something simple and something fun. The more and more people play these simple and fun games, the more and more they'll want to explore other games. The more someone explores other games, the more likely they are to come across something else they end up liking. Eventually, you get into the zone where you could be termed "hardcore." Which in the end, all hardcore really means is that you are aware of all of the different kinds of games out there and you tend to play a lot.
Overall, gaming for most will remain a hobby where "hardcore" and "casuals" alike will enjoy the fruits of the gaming industry. The true culprit, if you are worried about games losing quality, are people that will buy even the crappiest of games. Which promotes strategies that EA has adopted which is, rush development and add content later. This mix is dangerous because for the gamer it means lower quality of games as far as bug persistence goes. Also, less bang for your buck because you are forced to buy content in small packages, i.e. "expansions." Even then, since the developer is rushed, you'll lose ideas and features that would make certain games so much cooler. Take a look at Company of Heroes expansion, Opposing Fronts. Many features were promised, but Relic decided to rush development and the game release with tons of bugs and a lack of "promised" features. We were promised "ranked automatch" where people could play ranked games with arranged 2v2, 3v3, and 4v4 teams. This wasn't added until almost 1 year after official release. New game modes in multiplayer were also promised, but they failed to ever deliver on them. The excuse of "the edition of 2 races meant new game modes, because you get to play with those races" was given to the community.
If gamers were smart, they would promote companies like Valve who have a very good strategy when it comes to game development. They, on release, strive to have a quality game, with all intended features that have been announced intact. On top of that, they continually support those games and release new content and updates to them for free. Take a look at Team Fortress 2 at release, and what exists with it now. They've added new weapons to about half the classes, which they won't stop releasing updates until they have made the new weapons for all classes, and on top of that they have released at least 1 new game mode and an array of new maps since release. If EA had been handling that game, you would have dished out at least $10-$20 more for that added content.