I think the best way to think about FPS games, and games in general, is to seperate them into one more categories than we generally think of today.
There's not "hardcore" vs. "casual"
It should really be broken down into to something like this:
Family casual - (wii sports, kinect, kid games)
Young Adult Casual - (Call of Duty, etc.)
Hardcore - (RPGs, strategy, adventure, challenging games)
Bascially, FPS games have abandoned any sense of actual gaming challenge in favor of appealing to the college-aged crowd. Don't be fooled by the blood and guns, these game are most definitely aimed at a casual audience. A few will play them online so much and so competitively that they could be considered hardcore, but mostly it's a casual gaming experience - buddies getting together in the dorm room to play a few rounds, kids hopping online to 'pwn some noobs' after school.
Old-school gamers who appreciate a challenge may have to look outside of the big franchises to find a real strategic challenge - action games are geared for reflex challenges, not mental ones. And don't trust franchises that do offer challenge; if the find success they might want a bigger slice of the pie - and move into the casual market *cough*rainbowsix*cough*. If they don't, they may close up shop for good, not meeting publishers unrealistic expectations of a multi-million hit just because the game has guns.