Since when have stealth games been "niche," exactly?
Many of the stealth games released in the past 15 years have been multi-million sellers. The Metal Gear franchise has sold 30+ million copies. Splinter Cell has sold over 20 million copies, of which most weren't the watered down stealth game that was Conviction. The Hitman series has sold over 8 million copies of its' games. The latest big stealth game to be released, Deus Ex: HR, sold 3 million copies and the original sold near that too. Also, while Assassin's Creed is focusing less and less on stealth, there are still heavy stealth elements in the series, and it has been the fastest selling new IP of this generation!
Sure, when compared up against the sales of the most broadly appealing (and, not coincidentally, most bland) first and third person shooters that dominate sales charts these days, the sales of stealth games fall short. Does that make EVERY game that isn't a bland, catch-all shooter a niche title? No!
Just because most publishers have shied away from the challenge of making a stealth-heavy game this generation when it is easier (and more profitable) to make a shooter doesn't mean that the sizable audience for stealth games has suddenly vanished since the beginning of this console generation.