When you hit Delay and say no you don't want to actually do that, the game leaves you there at assembly for that pad. However if you hit Scrub and say no you don't want to, it returns you to the VAB Launch Selection screen.

I realize that the Scrub feature is older, so its behavior is what's original to the game, but I think the Delay behavior is actually better. If you don't want to scrub the mission, presumably you want to Assign it. I looked into this one, but don't see how the Scrub subroutine works.