It's not a cop out at all. Zobek was frozen and shattered, Satan was staked with the Vampire Killer, and they're both dead because the game says so, like it or not.
They can also be brought back in a sequel because "reasons" if wanted to. That's just how it is. I'm not saying you have to like it.
The game never said so. That's part of the problem. It
implied so, but gave no reasons why the implication should be taken beyond "because I said so". But even if Zobek walked out and said "This weapon is more powerful than God", it would still be bad writing. There's something called suspension of disbelief, and it requires more than absolute submission by the viewer/reader.
And you're right, it is perfectly within MercurySteam's power to write whatever shitty ending they want, but the "because they can" doesn't excuse the writing or the plot holes.
Dude, bad example. Have you watched Adventure time?
Yes? I've watched the whole series. It's a great show. What's your point?