I'm not sure what this is supposed to suggest/demonstrate.
Surely you agree that taxing (say) 20% of 60k is different from taxing 20% of 1B. Namely: the former has a much more significant impact on quality-of-life then the latter...
Figuratively speaking, what can you do with 747M that you can't do with 300M?
(Before the pedants inevitably pick this rhetorical question appart: the point is that it's not prima facie unreasonable to tax extreme wealth significantly more heavily than the median income)
Edit: the first of the pedants have arrived... sigh...
Invest in starting brain-computer interface and rocket companies
A more substantive question is how that money would be spent by the relevant parties. I would argue that Musk would spend it better than the federal government and as such, he should have it.
The idea that the argument is stupid should be presented with an argument as to why. For what it's worth, we have existing law in most (American) municipalities that support the argument that if Musk can be shown to be a better steward here, then he might have a claim to make, i.e., adverse possession, which is basically a claim against the state that says "I can tend to this property better than you, and my doing so provides social and financial utility to the state, to my would-be neighbors, and to myself as well.
To be sure, it's an argument in abstracts, but there's precedent enough that it clearly isn't an entirely meritless (or stupid) argument to be made.
That leaves him with $747M for the year.