That's only part of the trading though. Volatility isn't measured in that way either, so as volatility goes down-- or stays the same as coin prices go down-- Coinbase still loses money. It's usually measure in the % of the coin traded. So volatility could remain stable at, say, 2% but when the dollar denominated market cap is 50% lower than it was 6 months ago then Coinbase is collecting 50% less in fees. In order for revenue to remain the same, volatility would have to double.
People that had $10k invested already and kept it in the market now have much less than $10k. They may still have 10 ETH depending on when they bought in, but when they trade those ETH Coinbase is making 75% lower in fees than if they traded them 12 months ago.
Coinbase simply cannot make the same revenue in dollar denominated fees when the dollar denominated value of those trades goes down, and it has gone down a lot.