You would not expect a ridership reduction to have any significant reduction in operating expenses. Full trains costs roughly the same as an empty trains.
If ridership is down, I'd expect them to run fewer trains.
The problem of course then is that you create a whole in the bucket. Fewer trains -> BART becomes less convenient -> people choose other options -> lower ridership -> fewer trains -> less convenient ....
The problem of course then is that you create a whole in the bucket. Fewer trains -> BART becomes less convenient -> people choose other options -> lower ridership -> fewer trains -> less convenient ....