> the fact that their empires are now all they'll ever be

Eh, Mark Zuckerberg is 40. Facebook is planting seeds in some pretty ambitious places (VR/AR + AI). To put that into perspective, Elon Musk is 53 now, but he was ~40 when the Falcon 9 first launched for SpaceX and the Model S was released at Tesla. In June 2012, when the S was released, Tesla was worth about 0.7% of what it is now. Elon Musk was certainly rich, but no where close to the wealthiest folks at the time. Similarly, at 40 years old (21 years ago) Jeff Bezos was worth about 100th of what he is now. Rich, but it wasn't clear that Amazon would ever come close to, say, Walmart, in terms of market cap.

Mark Zuckerberg's empire still has plenty of time to grow.