I understand your point, and I sympathize when people shoot it down by citing some unique edge case where the logic breaks ('but what about my monthly Costco run?'), but I don't think mini-vechicles will be a solution for quite some time--not till there's a hyper loop-esque system in place or some other jetsons-like equivalent. I know it sounds defeatist, but human nature's resistance to change coupled with the complicated and slow moving bureaucracy behind safety legislation casts a heavy shadow on the single/tandem seater car concept.
That said, I think autonomous vechicles also allow for densification of roadways, and I think in context, 'regular' AVs are a significantly closer solution than minis. Elon understands human nature pretty well, and he understands that tackling autonomy with an otherwise [relatively] normal car is the quickest way to advance personal transportation. Trying to push a single/tandem type car right now would just too tough a row to hoe.
Btw, I'd buy a $10k single seater BEV in half a heartbeat.
I think that Musk contradicts himself when he expects owners to start sharing their Teslas. In another thread most of them indicated that they're not in favor of that. You might say, the pricier and the posher the car, the less likely it will be shared. So, you need a more utilitarian type of vehicle.
If you want to combine the sharing trend, more space efficiency and autonomous drive, you stand a better chance if you have a lot of compact or narrow 'utes' and just a couple of 'bigger cars' that are of course at your disposal whenever you need them. The reality is that most people need just one seat or maybe two seats 99% of the time.