My point is stablecoins give you choice to opt out of that. The only way to opt out before was very expensive
It fundamentally misunderstands how foreign exchange works, or how government backed currency works.
You cannot "opt out" of the local currency: period. It is the only currency which can extinguish tax obligations. And even if it wasn't government backed, you can't trade in a currency no one wants in the first place.
This should be trivially obvious from the observation that how much water a gold bar in the desert buys you is going to be pretty highly variable.
I assume you've never experienced hyper-inflation? If you have, do you think it's fair that you were forced into a hyper-inflationary currency? And, if given the means to, do you think it's fair that people _should_ have the ability to choose?
If you live in a place then you have to trade in whatever the local currency is. You didn't "opt in" to a particular stable coin: someone has to be willing to accept that specific coin as payment.
And they can't just exchange it to another: the exchange has to want to sell that coin in exchange for the coin you transact with.
And to interact locally with the government, you need someone who is willing to sell coins in exchange for the currency you don't want.
In practical alternate market economies, the only currency which trades tends to be USD and the exchange rate will be bad because it's a grey market. I would go further and posit that where crypto has any impact, it's people because it's a window into being able to hold USD.
Certainly the only question anyone asks about Tether is whether they actually have the USD to cover their position: no one wants a Yuan based see stable coin.
