2. it's got the advantage that you're using the same measurements as all your trade partners so people don't need two production lines anymore
3. it's got the advantage that people going into science and engineering don't need to build a whole new set of unit references because they've got the one which already works
4. the "huge expense" is pretty much made out of whole cloth for the purpose of saying you can't switch, the UK's metrication cost basically nothing except for road sign replacements which is why those are still imperial