if dirk gets to be the villain then that's just flatly a victory for him. he's becoming the villain, manipulating people, etc. because if he does nothing to keep the plot going then everyone he cares about dies (or is consigned to living in variations on Candy).

imo the only thematically satisfying conclusion is somehow proving that there was a viable third option to keep the plot moving better than "dirk does nothing and everyone dies" or "dirk manipulates everyone and positions himself as a new antagonist". this is not a trivial task! I don't think it's obviously-true that there is a better outcome that was up for grabs (indeed, if it was obvious, dirk would have realized that and not become the villain). however Terezi is well-positioned to do this eventually, since her Seer of Mind powers let her explore alternate choices people could have made (and this fits well symbolically since Mind is the opposite of Heart)