As said before, converting to USDT instead of real fiat doesn’t absolve you of paying capital gains tax, at least in most countries. Few countries like France specifically decided that exchanging crypto for crypto is not a taxable event, but that’s the exception rather than the rule.
And while they might not be able to find out that you haven’t paid your taxes right now, I believe most tax agencies will have this capabilities in 10 to 15 years. Remember, everything is stored on the Blockchain forever. If you get audited in 15 years, you’ll have to pay punitive interests and penalties over these 15 years.
And regarding USDT, it will remain around $1 as long as Tether remains solvent (and people think they are solvent). Otherwise it will go to zero. I personally don’t think they are solvent which is why I refuse to hold USDT.