One key difference is that you won't replace car speedometers overnight. If a 35 mph zone starts having signs that say "Speed Limit 60," drivers will get confused.
In the UK, all our speed limit signs are in mph but practically everything else is metric. It seems to work just fine. I'm not sure there'd be widespread confusion about changes on our speed limit signs either. People know what is unreasonably high, and they have km/h markings on their speedometers. It's why you don't often get tourists in Europe misunderstanding km/h roadsigns.