We didn't have the tech for nuclear bombs until we decided we wanted to blow up cities. We didn't have the technology to go to the moon until we decided we wanted to go there. A little more than ten years ago the Motorola Razr was the height of mobile phone design.

Stuff like NERVA or Project Orion stayed proposals not because they're impossible, but because there's nowhere we really need to get that fast to justify the risks/expense/fallout.

"We need to get to this interstellar starship" would change the cost/benefit ratio of developing the tech needed.