As demand for high speed rail increases, so will the price until it's on some sort of parity with air travel. What will likely happen is that air travel will become comparatively cheaper since it's more of a pain in the ass. Alternatively, airlines will just decrease capacity between the major cities that have high speed rail. Maybe airlines will do more to attract customers or maybe they'll just cheapen the service even further to beat out the cost of a train ticket. Maybe air travel will be like it was back in the early days, a luxury that most people couldn't afford.
Unfortunately for the US, we are stuck with things like Spirit and Frontier for the only way to travel cross country in a single day as cheaply as possible. I absolutely love business class on European high speed rail, it makes air travel seem like it's only meant for masochists.