BART is designed in classic rail style. Stations are "on line" which means the vehicle in the station blocks the line. No passing. Headways must be at least a few minutes.

You can pave over tracks and leave the rails in place, streetcars have rails in the middle of pavement. To do this well you want a way for vehicles to pull off the line and stop, with lots of space, and a way for vehicles to leave the line and go to city streets easily. BART is 3rd rail, which means it must not be "at grade" and that makes this harder, but not impossible.

Now, in the places they are talking about closing entire ends of lines, in Pleasanton and Martinez, that could be an opportunity, but the challenge is, they are out of money, so what pays for it? Maybe private entities?

So you pave those sections of line (leaving the rails embedded in the pavement in the event somebody wants to bring them back, though I doubt that.) At the stations, though, you have to somehow build bays for vehicles to pull off the line and slow down and get into the bay. That's going to cost. You also need a way to transfer off of an arriving BART to get into the robovans heading further down the line. Not hard to design if building from scratch, harder to retrofit. But if you could, if you came in on rail, the train would pull in and across the platform would be vans, waiting to go to each station, as well as vans for stations that don't exist yet, where the van will leave the track and go to a stop on surface streets. You transfer to the right van and off it goes. At the end of the line the bays are simpler, but for example in Pleasanton you need different bays. Most people would be getting into a van to head to meet an outgoing BART, others would take vans going to other local stations.