There are currently 11 federally designated high speed rail corridors with some segments that are more ready than others (e.g., Washington-Charlotte starting before Raleigh-Jacksonville). For the segments that aren't as likely to start by the end of the Teens, conventional service is best--assuming that Amtrak runs trains along that corridor).
Last February, The Transport Politic pointed out seven segments that would be hit with low ridership and 10 others with some connections that the original FRA HSR map missed. In order to solve the former problem is to provide conventional train service. Do the Coast Daylight and daytime Raleigh-Columbia-Jacksonville routes ring bells? The planned Daylight is the result of a partial revision that removed the coastal San Francisco-Los Angeles route from HSR consideration.
Once Amtrak loses some of its existing service to other companies, it should shift some of its equipment to other states for conventional Florida service to Orlando, Tampa, and Miami. Horizon Cars in the Sunshine State may be weird, but if Amtrak loses big time in the Midwest, it would need to add and expand service in the areas that it retains corridor and long-distance routes. Eventually, the state would order its own cars to expand service.
Any company that outbids and/or replaces Amtrak leaves an opening for the national carrier in which it could capitalize. Say that Spanish-based RENFE outbids Amtrak on the Milwaukee-Madison-Twin Cities extension while the latter company begins operating SEHSR service. No one can tell me that it's out of the question that Amtrak could provide a conventional daytime route between RGH and JAX. Such a route would serve three purposes: 1) it would relieve the Silver Star of overcrowding; 2) as a daytime balance, the times are more marketable with connections to and from local and express SEHSR routes in RGH; and 3) such a route would serve as a warmup for HSR service along the S-Line between the two endpoints.
Provided that Florida East Coast doesn't get tired of waiting for Amtrak and operates the route itself, Amtrak could get a California-like system going that could lead to a long-distance route using the JAX-MIA east coast segment that local, intrastate service would initially utilize.
Both of these examples would also put Amtrak in a better position to win other segments of a corridor when those routes finally meet HSR standards. Of course, that will also require a drastic change in Amtrak culture and for them to learn from any and all upcoming losses to foreign operators.