I don't know what other cities look like. I've ridden the subway in New York on and off throughout and past the pandemic. This week was the first week that seemed normal to me. I don't commute, but have ridden the subway at commuting times, and it's definitely a little more chill than it used to be. (Easy to get a seat! But the train does eventually fill up as you get farther into Manhattan.) Looking at the commuter railroads, which I think is the fairest comparison to BART, they seem to be 50% of their all-time peak capacity. Not ideal, and I'm not convinced that 50% of the New York metro will be allowed to work from home forever.
It will be interesting to see what congestion pricing does to people that used transit before the pandemic that drive now. When I worked in Manhattan, I had a friend that drove every day. He showed me the spreadsheet that he used to decide between a monthly pass on MNR versus driving, driving was ever so slightly cheaper. With a $20 a day charge to drive into the city coming in the next couple years, that might be the edge that the railroads need to make it a financially viable decision.
I always thought we should make mass transit free and see how the numbers do. If 400,000 people drive into the city every day, and 2,000,000 people take the train, it's clear where the tax dollars should go. But I think as soon as you charge someone that first $1 to take the bus, pretty much everyone decided "fuck it, I'll just drive". The costs are a little more insidious, wear and tear on your vehicle, gas once a week, etc. People count that as free. (For example, do you ever turn on electrical appliances without thinking about the cost of electricity? That's driving.)