They would have zero capital cost revenue stream at that point though. And it’s not like their car manufacturing segment would be worth zero. They could easily sell it or spin it off.
It’s only speculative as long as it doesn’t exist.
I disagree: They would have very high capital cost, since if they quit building cars (which is the main thing they know how to do) they'd have to buy the robotaxis from somebody else, not to mention the maintenance costs mentioned by @smogne41 above.
But seriously, do you really think that Tesla, a car company founded with the express purpose of converting America's and the world's ground transportation to electric, would quit making cars? Entirely abandon their core purpose? Based on one wacky comment from a CEO infamous for wacky comments! The idea is so preposterous that I cannot understand how anybody would think it was anything other than a joke.
Note also that if they sold their entire car-making enterprise, all their line workers, managers, and executives would have to go with it, because without them the business would be worthless. After that, Tesla wouldn't even be Tesla. It would be Elon Musk running an entirely new start-up company.
The idea is simply ridiculous. Tesla will not stop building and selling cars unless it goes bankrupt (unlikely) or the economy collapses due to a natural or man-made apocalyptic event of some sort (very likely if we don't take climate change seriously).