The way I explain this one to my European friends is: Do you remember the transition to the Euro? How you had to mentally convert prices at first? And how old people had more difficulties with it in some cases? Now, imagine that, not just for one unit, but weights, lengths, temperatures and volumes, all at the same time.
Well, you don't have to be so drastic. Just teaching metrics in all schools and requiring their use in higher education would be enough for the next decade or two. Only then you can start thinking about something as the petition asks.
Metric is taught and used in school (middle, high, university) in the US for any sort of math or science, leastways anywhere I’ve ever been… If I were using any kind of tools to measure things and make calculations, I’d use metric, but in daily life, viz. in speech, it’s easier for me to just use the familiar imperial—especially if I want to make myself easily understood to normal people.