Hi cats_pajamas,
Sorry for the late reply.
I based this tag change on the guidline found in the wiki page of highway tags in Africa (https://wiki.openstreetmap.org/wiki/Highway_Tag_Africa). The wiki state that highway=trunk should be used for “The most important roads in a country’s system that aren’t motorways. (Need not necessarily be a divided highway.) Note that such roads can be unpaved.” 

Given that in these countries National Roads (RN, NR or N, depending on country) are the most important roads, then trunk should be the most apropriated tag for these. As specified in the wiki page the fact that it is undivided and unpaved does not matter. Though I undertand the concern that some (or most) of these nationals are very bad unpaved roads and it look weird that they have the same tag as large paved road in other countries. However I agree with the wiki that what matter the most is the country classification and not the state of the road.

And I might indeed have missed some discussions around these topics. Please let me know if they are specific ressources I should check.