If any blockchain was forbidden, there would be a huge PR stress and infinite arguments about their viabilities.
Just let morons fail.
Tether may also have been allowed to proceed as a sort of test bed for the CBDCs which seem to be on the agenda. Now the narrative can be "the public has already demonstrated strong demand for USD-type cryptocurrencies, we just need to supply an official version."
It just takes a shit-tonne of capital, and it helps to have a functioning internal economy and flexibility to be able to defend against malicious agents.
I suspect there are other legal issues with creating an alternative currency in the us though, which is why these aren't currencies but securities. Which is still fine! A tethered coin is conceptually _similar_ to a 0-yield bond.
Presenting it as being "safe" without it actually being so is the problem here.
Disaster strikes when someone sees the mountain of money and thinks ”If we invest this money, we get to keep the gains”
This can be mitigated by using many banks, but it's hard to find banks willing to deal with cryptocurrency. So in practice all the deposits are at 1-2 banks.
A stack of paper USD in a safe would work, but there's risk of theft/fire/etc.