Emphasis on might.
Evidence suggests "a giant boat and some helicopters" is the more likely result.
Nobody wants to make sure the roof is shingled and doesn't leak but everybody leaves money for new stained glass windows or the organ that nobody knows how to play.
I’d prefer not to rely on them.
Many, many fewer than you assume.
Libertarians like to make lots of good-sounding promises to justify their favored radical policy, but it's bullshit and the promises don't pan out when tested [1]. By that point, the libertarian has gotten what he wanted and moved on.
[1] Or their policy was already tried and already failed, e.g. https://news.ycombinator.com/item?id=46876387, leading to reforms to fix the problems that they're now mad about and want to undo.
Philanthropy is anti-democratic; the people don't choose what is important to support, the wealthy few do. You can see that in the relatively poor public goods in the US, which has much lower taxes relative to peers.