Of course there are all kinds of little assumptions in our daily lives that would suddenly change and have to be addressed. In this case though, do you really need 170 sq. feet to store whatever you are carrying with you? Opening parking lots to development will generate enough value to provide a couple 12x18x72 inch lockers for each space removed. That still clears up over 98% of the square footage currently being taken up by parking spaces, and it would obviate the need to use cars for personal storage.
It might even lower theft since a big component of theft from automobiles is that a thief can look into a bunch of cars and only break in when something valuable enough is identified. And since lockers could be located inside where the people are, it would probably opportunities for would be thieves to have alone time with valuables.
> people like to forget stuff in their cars
If cars begin to be produced primarily for the purpose of transporting a single different person over and over again all day (with a smaller number of multi-passenger vehicles being produced for the relatively rare case of multi-passenger rides), then I imagine that eventually they would evolve a design that makes it more obvious when you drop things you are carrying. Currently cars[1] have all kinds of nooks and crannies for your wallet to fall into, because it's not that big a problem in most cases when you do that. But it would be fairly trivial to add a little fiber glass hip that sits flush with the seat. You would have to relocate the seat adjustment controls, but a lot of vehicles have already centralized those controls on the main console, and ridesharing vehicles would actually see enough benefit to consider such a change.
[1] this includes taxis because taxis fleets are made up of essentially consumer vehicles. This makes sense, because you always have at least 2 people in a taxi, so you can't really comfortably make it that much smaller. But if there were fleets of rideshared self-driving cars everywhere, most cars would only need to carry a single person, and you can drastically reduce the size of a one-person car without sacrificing comfort.
Edit: added third paragraph and the accompanying footnote