What's ironic about your statement is that it's this exact belief--that founders should remain in control--that has led to the widespread adoption of dual class shares, in which founders maintain control while ceding a majority of equity.
And the irony here is, by that same token, founders could maintain in control while not having such large equity stakes, and not becoming so rich.
So I do not think this is an argument for billionaires.
I don't think this is true of "proposals" in the plural. This is arguably true of the California ballot measure due, allegedly, to sloppy drafting.
But this isn't a fundamental aspect of wealth taxes; it's a choice. So I don't think it's a meaningful argument against wealth taxes in general.
(As an unrelated point, I think dual class shares are sort of bad and it makes sense to me to discourage them as a matter of public policy, but this doesn't seem like necessarily the best way to do it.)
It seems to me that the authors were, as you say, trying to close a loophole, namely that where I set up a Nevada LLC to which I hold 99% of voting shares but 1% of outstanding equity, put my assets in it, and just direct the LLC to spend on things I want it to--but in the process, they failed to realize that non-public super-voting shares (like Google class B shares) seem to fall into category (3) of section (c) (i.e. neither sole proprietorships nor publicly traded assets).
I don't think that's a great design, though it's still worth noting that it doesn't rule out privileged shares in general--GOOGL, to give an example, are privileged voting shares that trade publicly, and thus are obviously under category (1), and assessed per their FMV.
But in any case, I stand by my broader point: this is a specific detail of how the California proposal has been written; it's not a fundamental aspect of wealth taxes. Switzerland has a wealth tax that does not have this aspect, for example.
I think the Peter principle applies here; what you view as "competent people" are actually beneficiaries of systems, that they themselves can never independently create due to the nature of organizations themselves.
And the irony here is, by that same token, founders could maintain in control while not having such large equity stakes, and not becoming so rich.
So I do not think this is an argument for billionaires.