we'll see terrain improvements to existing airless rock/ice planets once atmospheres make it into the game, not before.
I disagree, I think that once atmospheric landings are a thing there will be little motivation to either complain about, or make changes to these airless rocks. FDev are open to the possibility that the colour scheme may not reflect reality, but I think it's unlikely that either the variety of detail or the complexity of the terrain will change much.
The bodies we can currently land on have only a height map, with less extreme variance than at the launch of Horizons, so no caves, overhangs etc. I guess this model will remain more or less the same as it is now (possibly with a slightly different palette), and any terrain modelling improvements will only go into Atmospheric bodies.
It could be that bodies that used to have an atmosphere but no longer do could receive an update to use a more sophisticated terrain map, which would be nice, but I don;t think this is likely to happen.
Horizons season content = height maps, Atmospheric season content = more sophisticated terrain modelling imo.
I'm not against debeigification, but I am in favour of scientific accuracy as far as the galaxy is concerned.