Depends what those said hardcore fans were looking for.
If MS abandons the hardcore and goes full Kinnect (which I doubt they will, as Kinnect has yet to be a proven sales beast and many games will still use standard controllers) then they can prepare to lose a lot of their audience.
Genres like Gears, Halo, any shooting game: I don't see MS losing the Gears or Halo audience. They will replace Gears with another TPS if need be, and Halo will still be there because of 343 studios.
XBox Live - I don't see Sony or Nintendo taking down MS's live infrastructure. If you think @Home is better thats cool but the majority doesn't. Online is still best on the 360.
Those that abandoned Sony because MS offers the same multiplatform titles - I don't see that changing. MS has picked up more and more multiplatform titles every few months (my most anticipated being Naruto Ultimate Ninja Storm 2) and more game companies are finding out going multiplatform is the most beneficial for the bottom line unless your game sells a insane amount (or your second party).
The JRPG audience - This is the only audience I could really see MS losing at the moment. At the start we had a lot of great JRPG exclusives (Lost Odyssey being the biggest one) but eventually everything became timed exclusives and everyone cought on. Looking at sales the audience seemed pretty niche and MS probably figured WRPG was the way to go when it came to sales.
I think if MS continues to keep doing what they are doing - while maintaing using the standard controller and looking for more first party exclusives - then they should continue to build a strong fan base, while really only losing the few that bought a 360 for JRPGS - hopefully Square-Enix and others make more of their RPGS multiplatform.