I'm in Australia, and was taught metric at school, but we only converted in 1966 (I think) when decimal currency was introduced to replace the British pound and pence we had. So my parents were all taught imperial, and growing up my dad referred to everything as miles, feet and inches.
So, when asked to estimate the length of something, I'll use either imperial or metric depending on whichever will give me the closest to a whole. eg. I'll estimate a foot, or half a metre, or an inch, or a centimetre, whichever is most appropriate.
If America "converted" to metric, you'd all have an extra method of referring to the measurements of everything. It's not going to stop everyone from thinking imperial. At least not for a couple of generations.