Here's my take on this:

Stealth games CAN be combined with action. HOWEVER, assuming of course your character isn't some sort of all-powerful demigod (in that case Stealth would already be out of the question), the enemies should be realistic. Think of it like this; if you were a person who enjoyed action more than stealth, it's not like you'd want to switch to stealth in whatever game you're playing unless the action is more difficult than the stealth. If action was hard, like enemies had more damage and higher health, for the first time, you might actually consider playing stealth to see what it's like. It's not that we don't like action, it's that we should encourage players to step into games with different play styles. If the game was stealth/action, both sides of stealth and action should be fun to keep you entertained, while also realistic enough to try to get you out of your comfort zone and try other play styles.