As someone who's getting a custom rim brake frame made, this thread is making me nervous.
I'm just banking on the fact that, as others have said, there will be enough demand for rim brake parts for at least a cottage industry to remain.
What I'm curious about is whether or not the next generation of group sets will still come with a rim brake variant. Would hate for whatever groupset I select for this bike to be the "best" and final groupset it ever sees.