Comment by Ringz
The calculation in the metric system would not necessarily be more complicated, but it would be different because the reference points in the metric system are not directly aligned with the geography of the Earth.
"1 knot is about 100 ft/min which is very convenient for descent at a specific glide slope (i.e. for 100 knots ground speed at 5% slope you want 500 ft/min descent rate). Standard is 3° which is about 5%."
You are right. It's an easy calculation. But I would say its easy because its historically based on imperial units. Its easy to think about easy calculations like this in metric units like:
A 5% slope means descending 1 meter vertically for every 20 meters horizontally.
The gradient thing would work if ground speed and vertical speed were both in m/s, but km/h is more common in metric for a ground speed. You don't usually think in terms of hours during a climb/descent!
Glide slope of 3.6% would fit nicely though. Then, 100 km/h ground speed goes with vertical speed 1 m/s.
Metric navigation would use the fact 90 degrees of latitude is 10,000 km.