I think that having the world gen settings of a Ruins only server would be a pretty great idea, and I'm not quite sure how hard that would be. You would only need to get rid of the non-caves-like environment and biomes and presets. Keep the sinkholes the same way, just replace everything in the biomes apart from ruins material from spawning. I acknowledge the fact that these would make the ruins-only and caves-only servers smaller. I've heard people of linking multiple dedicated servers together (more than 2). I haven't seen them or hopped in them, but maybe if they had a setting of having a Caves-only server and a ruins-only server one could link all 3 of the servers together to get basically DS completely ported into DST.
TL;DR I'm not sure how hard it would be to have the options to have a caves-only server and a Ruins-only server and I'm not quite sure why the devs didn't implement this. Anyone want to give me feedback on why my ideas are wrong? Or has this already been implemented?