Yeah. I guess the latter alternative would be less of a letdown to fans who would hate to see Cooper die, but I guess they could've stretched it as long as they wanted with 'evil Cooper' being the new baddie over many episodes though, although it probably would've killed the show as he was/is such of a hero and sort of the main character of the show, so it'd made no sense to get rid of him (Also: for the sake of consistency, as it's told earlier in the show that 'only the pure' would be able to pass through the Black Lodge unscathed, it makes little sense that Cooper would've failed that test and left it evil- precisely because he's so pure and focused in everything he does, he of all people would've been strong enough to survive the stay in the Black Lodge intact and without any damage...) The second option with the real Cooper unspoiled and intact, trapped somewhere while his evil doppelganger impersonates him and wrecks havoc while he tries to come back and defeat it/save things IMO makes more sense as a plotline as it could be dragged on for a long time and also twisted around in so many ways- for instance, if he managed to defeat the doppelganger once and it seemingly died/disappeared, there's nothing to stop them from bringing it back a second or even third time, explaining it away with 'oh, the evil forces gained strength so they managed to conjure up another doppelganger and set it free out in the world...
again! *sinister music* I honestly think they could've done much more with that than option A (if coop went bad, they'd have to kill him off to make things right again and so the whole show must've ended with him and no more seasons could've been made, and i doubt the viewers/fans would continue to watch the show if it went on without him...) But I've already lost the red thread in my post now, sorry- forgot what i was going to say