As you say when there is only fighter in town, competition withers and dies.
Only one fighter in town? What about the F-15, F-16, F-18, Typhoon, Rafale, Gripen, Su-30, etc? Seems to me there are currently lots of fighters in town. If (a huge if) the F-35 captures the whole market and pushes aside all the others, that will be because it has out-competed them. Personally, I don't see that happening. Especially when you look at the specs for the Trainer-X (T-X), which will have far superior turning performance than the F-35. For anyone who does not need a stealthy fighter, that airplane will likely be eye-watering.