it makes it trivialize easy to issue any number of tokens (trillions? quadrillions? 2^64?) and then trade with them.

These are all fully visible. Gift cards we have no way of knowing how many there are.

Gift cards still have the limitation that you need a network to accept them,

Same with tokens. Only certain people will redeem certain tokens.

and they are commonly understood in accounting practices so retailers need to account for them.

Public blockchains are a triple ledger system. They are self accounting

How can we be sure that some random stablecoin is really backed by currency 1:1 like they claim?

Great question. Same with gift cards. Same with bonds.

The largest stablecoin in use today is Tether, which claims to have over $100B in circulation, all “backed 1:1”. But every audit they have released has been deficient in some way.

Oh yes. I wouldn’t touch Tether with a 12ft pole. I’m not saying stablecoins are good. I’m saying they are no different to what currently exists.