Billionaires are still people. If they become too unsatisfied with their treatment, then they will do something about it. Just like everyone else.
Since I assume you're not literally suggesting that billionaires will arm themselves and I dunno, patrol their golf courses and beaches with rocket launchers, I suppose you are suggesting they buy some island and hire private forces, and I dunno, next they have to move their factories or offices there, and presumably hire workers and provide healthcare and education and retirement...
I guess if billionaire island seems like a really good deal, workers might move there. Without knowing more about (and assuming it doesn't just get annexed) it kinda sounds like Dubai.
Or sure they could try their luck in some other country if they feel safer there. Maybe somewhere that's happy to provide "inequality services" / free guard labour for them.
Re: billionaires taking over government so it works in their interests, this has basically already happened and these kinds of wealth tax proposals are a way to balance billionaires existing with the effort needed to sustain them.
And seriously when you look at the alternatives 0.5% (say) doesn't seem like such a bad deal.
But since this is really a "should" question (aka about values), theres not really a right answer. We can mostly just talk about fairness, of which there are many different conceptions.
My take: Society has to work hard to preserve the billionaire's wealth. You can't be king without an army, and billionaires exist because of laws, police and a social fabric that lets them keep owning so much stuff. You would otherwise have to be a warlord to keep your hands on so much, and that has its own costs and risks.
Perhaps we could look at it as a fee or compensation for the aggregate social guard labour required to preserve the social order that ensures the safety of the billionaire's assets.