I don't see anything changing anytime soon unless a nice thick legged horse starts making headway. Probably not going to happen as TBs aren't known for their thick legs, but rather for having more slender legs.
The real problem with the length of career (which isn't really a bad thing after all) is that the top breeders don't have to make their one racer last forever. They have dozens to enter into races. Many racers of olden days had to not only last longer to get back the money sunk into them but also run it all, short races, long races, wet tracks, dry tracks, etc. Now they have one for short races, another for longer races, and another for races on wednesdays
Its also not tracking what happens at "retirement" or what it defines it as. If its just not racing thats one thing, but many racers who don't make it go to slaughter, or long careers as riding horses.