You know what, I can see what you mean about that last part because when Rose was saying that she was afraid to ascend at all because what if the version of her that persists afterwards isn't "her" anymore, Dirk says, "Who cares, she'll be better." In response to her worrying about the metaphorical death of her other half.

I think Dirk is definitely a person who wouldn't care at all if he had to "kill" the other halves of his friends and let them ascend that way. However, I do think you also raised a good point by stating that Rose and Dave's other halves are still out there and don't necessarily have to merge with them for them to be "whole". That begs the question then what will happen when they really do meet their other halves, will Dirk really just let it go and not care that those other halves exist at all? Idk...Especially not when those other halves want to literally stop him.