And yet most stable coins have maintained their peg for years now, despite many of them having gone through 2 large bear markets. If 80%-90% down and still pegged doesn't disprove "impossible to do on the downside" I don't know what could.

Clarification: I do think Tether is not fully backed and is likely to depeg some day, but the assertion that it's "impossible to do on the downside" is clearly not true even for Tether. And it's very clearly not impossible to keep a peg if the peg is fully backed, which may be the case for some coins (without naming specifics). DAI, backed by verifiable collateral, has also done extremely well through 2 bear markets.