So this wouldn't be helpful as an incentive to get them diversified?

That's an arbitrary incentive that's not particularly useful. And in any case, it forces one to relinquish a significant partial ownership in their own company just to be able to afford to continue owning the rest...all so that we get them to diversify their portfolio?

This sounds like the exact opposite of what the US as a whole wants to occur.

Would Elon be interested in selling his stakes and diversify? Would he be lose control over his company to retain his wealth?

This comment was flagged for some reason, but it's entirely valid.

If Elon Musk's goal was ROI maximization, he has no business maintaining ownership in SpaceX and Tesla — he ought to just dump his billions in high yield ETFs.

Fortunately, that is not his primary goal, and (at least in the US), he is able to continue to pursue success by maintaining ownership in his companies.