I believe only a few countries use meters and other SI units in air transportation, including India and China (please correct if wrong).
Using kts and NM in navy -- built from the length of the 1' arc on a meridian -- simplified nautical mental arithmetic, and likely also helped at the beginning of aviation.
Current ICAO recommendations
In Annex 5 - Units of Measurement to be Used in Air and Ground Operations, ICAO allows 3 non-SI units (knot, foot, nautical mile) to be used...
as alternative units because of their widespread use and to avoid potential safety problems which could result from the lack of international coordination concerning the termination of their use.
... but overall states in the same document that only SI units should be used, the same units for all members. Nevertheless the termination dates for alternative units have not been specified:
It is intended that the use of the non-SI alternative units [...] will eventually be discontinued in accordance with individual unit termination dates established by the Council. Termination dates, when established, will be given in Chapter 4.
Case of vertical measurements / altitude
Vertical measurement in meters may be the more complex due to the existence of flight levels in feet, with the full system of vertical separation based on FL.
This topic was discussed in Using feet vs meters for altitude?. The current question is not a duplicate of the latter.
Other non-SI units
Other non-SI measurements seem more easy to remove:
- Horizontal distances in feet and nautical miles may be expressed in meters, as already done for runway lengths. It is not as important as it was to mentally translate a latitude offset in NM.
- Atmospheric pressure expressed in Hg column height may be expressed in Pascal. Many pressure instruments can be already calibrated both in inHg and hPa.
- Speeds expressed in knots (sometime in statute miles per hour) may be expressed in km/h.
Wouldn't it be useful (and safer) in the long term if we could remove most of the imperial units from new aircraft instrumentation, procedures, ATC, documentation and maps (at least for anything other than altitudes)
Just asking what is the current view from the air transport community itself.