When I was learning the penny dropped when I realised that if I heard "feet" that was up or down and when I heard "metre" or "nautical miles" that was in front or behind. i.e to differentiate between vertical or horizontal distance.
That's an interesting theory, and very personal. I'm not advocating anything as I was so used to the current units; but it no longer matters, professionally. If west EU measures visibility & runways in meters, and metrication has become the European norm outside UK veggie markets, and east EU uses meters for altitude & wind speed, and I've flown French GA a/c with kph ASI's, it would be possible to go fully metric in EU if range was also quoted as km's. However, the might of EU, west & east including Russia, would lose out to the power & influence of USA, China, Japan and southern hemisphere, and the status quo will remain. So metrication in our industry, is a part-time affair and at the end of the day it's only numbers.
Hap became possible because it was at no cost to instrument manufacturers. The same with changing Kelvin to Celsius. There is the battle between Watts & Joules, that may still be on going. Not good for anglo-french relations. Fortunately Newton holds sway in one field.
And who gives a peck or a bushel about it all anyway. 'Chain' them up in the 'yard' next to the horse with 14 'hands' and a 'cubit' of mane, drinking a pint and telling the rest of the world to go forth & multiply.
Happy New year, EU version, but at differing times.