We only tag what can be verified on the ground, and since there is no 140 speed sign so we cannot put maxspeed=140

If there is no explicit maxspeed, we put the implicit maxspeed BG:maxspeed.
https://wiki.openstreetmap.org/wiki/Key:maxspeed#Implicit_maxspeed_values

If we wrongly put 140 and the law changes, then we must re-tag all the country. BG:motorway keeps us safe from law changes.