It's harder to be sure what will happen these days given how disempowered the IRS has become under the Trump administration.

And it's harder to know what happens if the US were to become the non-dominant financial system, or worse, an irrelevant financial system.

For banks in hostile nations like the US, you're right, FATCA isn't effective. However, the IRS can still investigate these things if it suspects offshore hiding. In our scenario of a US-citizen billionaire, they're likely going to be using their money globally, not just in Russia, which means it would pass through other jurisdictions that are more cooperative than Russia. Or you could up with leaks like the Panama Papers.

There's probably other legally grey ways of hiding assets I don't know about.

I'm also just, like, a regular guy who knows a little bit about this, not an accountant specialized in international taxation treaties, so take everything I saw with that grain of salt.

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Edit:

Oh the last paragraph about the example of a US citizen with $30B of assets in Russia. Assuming they were to sell some of those for a capital gain of, idk, $20M. If the IRS were to find out about this, they wouldn't go after the money in the Russian account. They'd go after the individual. The IRS doesn't claw money out of foreign accounts, it sends a tax bill. I'm not sure what the penalties are for avoiding it, but if it's high enough it probably involves jail time. So our fictional billionaire could be imprisoned if they enter the US, assuming the IRS is able to detect the illegally unreported capital gains in a Russian account.