I do think governments all over the world work hard to find some way to tax their foes and let their friend pay a pittance. At the same time, they don't care any bit about the people with just barely a surplus, that end up collectively paying most of it.
... then they'll leave the country. I don't think you've thought this through.
Other countries could follow suit, though the USA probably has the easiest time enforcing due to its global hegemony.
If they flee to other countries and illegally circumvent the law then that's where the US should start flexing it's muscles.
2) Where are they going to go? Somalia? Other countries are going to tax them at least as much. These rich people aren't idle rich, they're rich because they're working in some capacity. You can't do that in Somalia. They get their wealth from being located here. That means they can be taxed here.
Sell it. Or don't, it's their choice. At some point real estate holdings aren't worth it if your host country decides it's entitled to 95% of your wealth.
>Where are they going to go? Somalia? Other countries are going to tax them at least as much.
There will always be another first-world country more than willing to undercut other first-world countries for the opportunity to host wealthy citizens. It's the same principle that gets us to where we are today, with states competing against each other to give tax breaks to corporations to entice them to set up shop.
It's funny how there's no end to people complaining about how the wealthy always seem to get out of paying their "fair share" through loopholes, but THIS TIME, we've got a foolproof plan to redistribute their wealth that they can't weasel out of! Sure thing.
The US example is usually to remove Social Security, food stamps, HUD, unemployment insurance, low income tax breaks, etc which means close to zero net cost but a massive shift in benefits.
Overall I think the driver for UBI is more about complexity than welfare. For example without changing revenue you could eliminate the need for tax prep. However H&R block for example really loves their billions per year in waste. So in many way's that's the context UBI should be judged not just it's direct impacts.
1: the average income is significantly higher than the median income.
It looks like the highest marginal tax rate in the UK is 45%. You could raise that higher, of course - but it's not clear how much, and if doing so would actually raise revenues - where the UK is on the Laffer curve.
https://en.wikipedia.org/wiki/Laffer_curve
(Clearly, if you raised it to 100%, it would lower revenue - you can't tax the rich if you don't have rich people anymore. And if you're at 45%, how much higher can you go?)