On the contrary, Amtrak represents the exact scenario that OP was asking about. It was created following the complete collapse of all the major passenger railroads.
The worst case is they could all merge into "Amwings" and those who choose to AGTOW would go bankrupt. I doubt it. Consolidation down to fewer major carriers per country would happen first because it's not like HYPErloop is going to put airlines or air cargo out of biz anytime soon.