So why do we measure:
speed = knots,
route distance = nautical miles.
altitude = feet
visibility = metres
wind = knots.
I noticed in Flight International, many years ago, aircraft range was published in Km's??? and speed in kph, but the aircrafts instruments and FMC's still used Kts & nm's. Duh!
And there is huge resistance in EU to use Meters for altitude instead of feet; wind in m/s; like eastern europe; and visibility in feet like USA.
Thus I think there is more to sort out in our 'supposed metrication' standardisation than Hpa v Mbar. It's only numbers, anyway. We managed to change, and get used to - slowly - runway lengths changing from feet to metres, but I found visibility in feet difficult to assess. All you had to do was compare the numbers to the minima, same as metres.