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.