Well, how about, "any system that gives another outcome is not really playing the genre Lovecraft wrote"?
Why not? Any reason that high level D&D characters can't have an old junker ship to bump him on the head with?
Cthulhu stats also appear in Bestiary 4, which means that he'll probably appear in the PRD soon, too.
EDIT: To further clarify my thoughts here, as I recall, what would have been the equivalent of a low-level Expert class NPC bumped Cthulhu on the head with a boat, and sent him packing. High level D&D characters shouldn't have found that any kind of challenge whatsoever.
This was not unique to Lovecraft's writing. A posse of federal agents put a decisive end to the lair of the Deep Ones in
Shadow Over Innsmouth. A handful of old professers with access to a banishment ritual of some kind put a decisive end to the monstrous "half-brother" of Wilbur Whateley in
The Dunwich Horror. Randolph Carter flouted the designs of no less a villainous team-up than Nyarlathotep and Azathoth simply by waking up in
The DreamQuest of Unknown Kadath (although, sure, I'll concede that he didn't kill them in mortal combat, exactly.)
The notion of defeating Mythos entities is not in the least antithetical to the writings of Lovecraft. And it's even less so to the writings of many of the writers of the original Lovecraft Yog-Sothothery circle. For some reason, amongst gamers an apocryphal interpretation of Lovecraft's writing, that isn't really supported very well by many of the "primary sources"--(i.e., his actual writings) seems to have a lot of currency. It's even arguable how much Lovecraft himself was trying to push this posthumously constructed paradigm of cosmic horror in most of his writings. Just because he referred to it once in an essay on how to write a successful horror story (from his perspective) doesn't mean that that's the sum total of Lovecraft's ouvre.
