Let's be honest here, "hardcore" is completely subjective. What some may consider hardcore others may consider cheap, or uninteresting, or etc. I mean I saw a show where they were doing stuff about who was the best gamer, and one of the challenged was ROCK BAND. That's a party game, and certainly not hardcore. It's fun, in short bursts, but not hardcore. And had no business in a gamer show.
I've been playing games for a long time now, (more than two decades) and the fact of the matter is gaming is way more accessible now, and it's a huge industry, even with the rampant piracy. Back in the day gaming was a very small group of stereotyped individuals who managed to get a game system and games for it, and bragged about their success. But now that the tech is so much cheaper, and the widespread internet access, anyone can be considered a gamer. Companies are only out to make profit. That's their job. If that means making their medium easier and more accessible to make more profit, they will do it.
Having said that, "hardcore" gaming, if by hardcore you mean extremely difficult, is far from dead. With Dark Souls having shipped 1.5 mil copies, and the other more difficult games people have already mentioned here, it isn't dead. There were never many "hardcore" gamers, and there never will be. More often than not, if someone can go through on easy mode, they will.
But let's be realistic. Unless you're playing the original Ninja Gaiden or Final Fantasy with only white mages, in a house that's on fire, while you are punched in the groin repeatedly and have a mouthful of bees, "hardcore" doesn't really apply to gaming. What may be easy for some might not be easy for others, so you can't really have a true definition of "hardcore".