No one is forcing them to retire, I for one would be interested to see what these productive people would come up with when they shift their goal from making a great profit to making a great social impact.

I think this assumes that only Certain Blessed Special People could create Google, Microsoft, Apple, Tesla, SpaceX, and so on. I think there are a lot of people out there who could start and/or run one of these. The population of entrepreneurs is limited by the number of potential commercializable ideas, not by the number of people who can do it. If people just stopped at $50M, it would open things up for even more entrepreneurs to make shots like those companies. Currently, a small number of already-rich people are using up all the economic oxygen in the room trying to get richer.

People can continue to be productive without meaninglessly adding more digits to their network.

Or maybe others would be highly productive if the current people retire.

Put the cap on assets. They should be able to make all the money in the world as long as they spend it too.

Exactly. We tell Elon Musk "hey buddy, you did amazing with Paypal, but that's enough, we're forcing you to retire."