A stablecoin isn't permissionless/borderless. As said, Tether can decide to blacklist certain Ethereum addresses arbitrarily, and it has already happened. It's just one entity. You may not have to hand them over your identity to register, but it doesn't go in your way. You're under their own rules; not yours as in bitcoin. You someday might not be "worthy" enough to have access.

Market capitalization is pointless in cryptocurrencies, let alone in stablecoins. It doesn't show the money that's inside the system. Tether can mint a quadrillion USDT, but there's no real money created other than unlicensed IOUs.