That doesn't impact the value of Bitcoin. When you exchange USDT for Bitcoin, the loser isn't holders of Bitcoin, the loser is (potentially) holders of USDT. Exchanging USDT for Bitcoin doesn't create new Bitcoin.

Coinbase, as a marketplace, engages in the exchange of (potentially) bad tokens like USDT, as well as tokens like BTC that are provably finite. All of this is orthogonal to the fact that there is nothing hypocritical or internally inconsistent about holding and using Bitcoin or Ethereum and using Coinbase.

When banks use bitcoin instead of fiat, they could in fact create more 'bitcoin' in the same way.

You wouldn't own the bitcoin, just as now you don't own the real fiat. There would just be a number on your bank account that says how many bitcoin you own, but it's multiplied the same way as they do now.

It all comes down to the % of reserve they need to hold.