I wouldn't call those stablecoins as cryptocurrencies just because they are listed in cryptocurrency exchanges or recognized by some entities. USD backed tokens have nothing to share with the main concept of cryptocurrencies as defined by Satoshi when inventing the Bitcoin. They are fiat backed cryptocurrencies which means they have value derived from fiat money and this is the main reason why they are called stable (which is not true stability). In addition to it could be frozen by the devs who invented it, which makes it riskier than any other cryptocurrency in the crypto industry.
PayPal recognizes stablecoins, or youtube use them to pay content creators, all of them couldn't make those called stablecoins as real cryptocurrencies. They are just a crypto form of fiat money.
It's true that it's not a real crypto. if we're going deeper into the technical aspect, It's not even a proper coin but just smart contract code living in the blockchain. It's basically just an ERC20 token but regardless stablecoin being accepted by many companies means we can easily create a swap route that uses real crypto like ETH to make settlement in purchase to a merchant that accept stablecoin.
It's an indirect crypto adoption because it opens up another route. You basically don't need to withdraw to bank and add another middleman fee. That's why this adoption could be beneficial for crypto as well although we know stablecoin isn't real crypto.
I use stablecoins for everyday payments in small transactions since many locals know about USDT and start using it. I trust the use of USDT but not the USDT itself. For all the reasons, it can't be the safest way to store many for a ling period as investment