I believe that is angle Graham was coming from. At a certain point you wouldn't be able to afford to control your own company. You would HAVE to sell your shares to cover the costs of the tax.
It is both an over and underhyped field depending on the area you are looking at.
HUGE difference between consumer self driving car [Everywhere, at all times] and the Machines as a Service [Geo, time, weather-fenced operations][1].
Waymo appears to be at the head of the pack; Sacha Arnoud, Director of Engineering for Waymo gave a talk a few weeks ago at MIT[2] and gives a good idea. We are about 5 years out until these start rolling out as MaaS (machines as a service). Probably 10+ years for level 4 highway operations for consumer models according to Frazzoli.