I propose calling it Implementation Success Bias, or maybe just Expected Success Bias.

There is a much more serious subset of this when people maybe-consciously look at the worst possible outcome of their opponent's view, and then claim that their opponent is eeeevil and specifically desires that outcome.

I have to say I agree with being annoyed at how whenever people have a Great Idea, they tend to COMPLETELY ignore the actual implementation of the idea (often jumping to considering problems once the idea is implemented).