That was kind of a stupid way to go about it. "Oh, we only need to kill one person for this to work." Most of history requires more than one person, and it's not all that easy to change things. The movie got that right, sorta, with whole 'going back is what causes him to be born in the first place' thing, but if things were really altered, shouldn't she suddenly not have a kid and be back at her apartment like nothing ever happened? These are the things that should keep me up at night.

The unitards don't need missiles. Plasma guns exist.

That's a crappy solution. But. I guess it doesn't matter. Life's what you make it.