Is it actually retro style over modern tech, or is there just not enough interest to support a thriving selection of high-end modern parts?

That's an issue which might take a while to resolve even if it turns out to be a good idea, since it's sort of a chicken-and-egg problem where what would be the target demographic consists of a high fraction of people who are sort of horrified by the idea. Right now, 27.5 MTB rims might well be the most viable high-performance 650b option.