Well, there's no doubt that M-rated games are leading the sales charts, but I'm pretty sure you need more than the revenues from just the top 10 games to turn a profit. Basically, Microsoft's (and its publishers') concern should be that its mega-successes tend to be of a very specific type - AAA, huge marketing budget, M-rated "hardcore" games. If you're a publisher/developer who can afford to produce that type of game, then it seems you're golden on the 360, even if Microsoft never sells another system (excluding expensive flops like Stranglehold, K&L, etc.)
But what if you're not a mega-publisher or well-funded developer - i.e. the vast majority of developers out there? What if you want to produce a family game? A puzzle game? A game that mainly targets female gamers? What if you don't have a huge budget? Where do you go? Last gen, most of those developers and publishers went to the PS2 (or the GBA), because it was so dominant and widely-appealing that it covered multiple demographics. This gen it seems they're going to Wii and DS, and if Microsoft ever wants to approach the PS2's success, it'll have to fight for that demographic as well (which I realize it's trying to do with Arcade and stuff like Scene It, but it takes more than a repackage and one casual game to change the perception of the system).
Even if these small and mid-level developers aren't as high-profile as the EA's and Activision's of the world, and their games never appear in the top 10, they're still important to the ultimate success and profitability of any console.