That's only because of the cost of replacement is so eye-wateringly high.The automotive approach won't work for GA, because the useful life of a plane is an order of magnitude longer than a car's life.
They could require that all new planes use only UL, and in about 40 years we'd still have more than half of the fleet running LL.
Think of it like Cuba in the 80's. The streets were filled with 1950's US automobiles -- not because the whole country were classic car fans, but because they could not get US-sourced vehicles to replace their existing stock at a time when the US dominated the car market worldwide. So, when something on their existing car broke, they repaired it because replacement wasn't a real option. That's how you keep an older vehicle running -- you fix whatever breaks on it.