so far, it seems to me that BIAB can generate accompaniment with RealTracks that is indistinguishable from a live session recording (unless I do something goofy like adjust tempo or scale into a range a real instrument could not reach.)

Can MIDI do that using a high-quality enough synth? Or is it always gonna be detectable as MIDI?

I am not disrespecting MIDI or those who use it. Just, for my personal tastes, it does not seem to deliver what I am looking for. But maybe I'm wrong and having the right synth would make all the difference. That's all I'm trying to find out!