That's a really good point, more demand for the limited number of transaction space pushes up the tx price, incentivizing mining. Currently though, the majority

That's a really good point, more demand for the limited number of transaction space pushes up the tx price, incentivizing mining. Currently though, the majority of the block reward comes from inflation not transaction fees. With BTC price so high, it's likely that stopping transactions completely would not cause any miners to drop off as mining would still be profitable, which is why I say there's no marginal cost. In the future, fees will necessarily become the dominant compensation as the inflation rate decreases, so this will likely stop being true. This is pretty far off though, and it's not clear that the environmental issue will still exist either due to a change in mining or just wider use of renewables for mining.