I am definitely curious to see how many here are willing to share, started a poll over in the Autonomous section to find out:
Would You Be Willing To Share Your Car?
I wouldn't anticipate that folks buying Model S/X cars are the demographic that will be sharing. As a future Model 3 owner, I likely won't be participating either.
However, what Elon is proposing is extremely forward thinking and lies in contrast to Uber's future model, which I presume will have Uber owning all their own cars. Elon's plan is to essentially crowd-source autonomous travel, in the same manner as how Uber crowd-sourced taxi's. If the economics are done right, you could have people buying or leasing a 2nd, 3rd, or 4th Tesla
solely for being autonomous taxis. Think about it. Tesla takes 30% for running the network, you get 70% (standard mobile-app arrangement for the sake of argument), and if you're in a hot market, you might break even in a year and then run a profit after that.
The biggest problem though, is that you would need to build the charging infrastructure before you sold any trucks. Building Truck stops every 300 miles(you would need to provide enough for truck to reach off route destinations) along trucking routes before selling any trucks would be a huge cash outlay. You would have to spends 100's of millions or even billions before you made any revenue.
But the truck stops already exist. The Tesla chargers, by and large, already exist. And, all of this is going to happen eventually anyway. In fact, the trucks will eventually be fully autonomous AND self-charging. So, who's in a better position to create that future other than Tesla?
The real issue, as you pointed out, is the battery density/weight problem, which will see continuous improvement as batteries improve.