OpenPilot and comma.ai are very close to a bolt-on after-market solution; not FSD 14 quality but you can get a lot of autonomy and intelligence with a $1000 Android device and thirty minutes for installation.

It isn't truly stick-and-go, you need to have existing ADAS and other sensors already in the vehicle. As people are now holding onto cars for an average of 15+ years, the bolt-on kits are about 10 years ahead of the average driver's car.