I could understand the argument, but as long as the system collects KYC info (which all legally operating payment processors outside of a few countries, already do) it is legal. And using stablecoins instead of volatile coins like XMR, BTC, etc. eliminates the possibility of extreme value fluctuation (and therefore wildly changing prices).
I don't know, you are probably right, but I can't wrap my head around exactly why. Even volatile coins like bitcoin have storefronts accepting it now. I don't know, but it sounds like a decentralized currency is the most efficient way to handle payment processing without actually needed a dedicated processing entity. If it's on a blockchain that already exists, wouldn't the processing be handled by the blockchain the coin exists on?
Like I said, I'm sure there are plenty of reasons this hasn't been implemented yet, stuff way more complicated than I am considering, I'm just some lowly non-cypto dev, I could easily be wrong about everything. Seems too logical to actually be this simple.
-EDIT-
My biggest personal argument against crypto is privacy issues. How payments would be obfuscated would need to be really good in order to ensure tracking wallets doesn't happen. Tracking bitcoin movement is extremely easy if you have the wallet address of anyone who receives payment (e.g. the storefront itself), and then you can see payments received by the storefront, amounts, wallet addresses of people who sent payment. I could see privacy concerns being a huge issue, atleast with bitcoin. I hear Monero is much better as far as privacy goes, but I don't know enough about it to confirm.