(I'm fairly bearish on the prospects/likelihood of self-driving cars in my lifetime, but that's orthogonal to the point above. If they exist, we might not need to build mini-tanks anymore.)

in this vision for the future, is it still necessary to chisel out 6ft on every surface street for humans to ride self-propelled bicycles?

It's tough to see how to get down to 6 foot wide lanes when there are so many vehicles on the road which are wider than 6 feet. A Ford F-150 (thirteenth generation), which is the best selling pickup in the US, has a width of 79.9 in = 6.6 feet.

Or, consider a family with 2 parents and 3 kids, where all three need car seats, plus room for strollers, extra clothes, and so on. I looked at top DDG hits for "car for three kids" and all of them were over 6 feet in width.

Then there are service vehicles, like garbage trucks, which are well wider than 6 feet. (https://www.reference.com/vehicles/size-typical-garbage-truc... says a popular model is "approximately 102 inches" = 8.5 feet). Where do they drive without blocking two lanes of traffic.

It would also be impossible to use by any RVs. https://campergrid.com/average-rv-length-width/ says "Travel trailers or a camper trailer on an average are around 20 feet in length and 8 feet in width". Others are larger; "Class A RV width stands at an average of 9.5 feet".

Or conversion vans for people in wheelchairs.

These are all solvable, but there's a lot of existing - and expensive - systems in place which would need to be changed to make that vision come true. At the very least you need to wait until people who don't have narrow self-driving cars no longer have the political influence to keep others from kicking them off the road.