To be completely fair to Musk, FSD is not what got Tesla to where it is today. It also isn't why the stock was overvalued; that was more Musk's mystique than any promises of FSD specifically.
No, Tesla's stock is taking a major hit because Musk is acting like a crazy person. If SpaceX were publicly traded, it's stock would be tanking too.
Musk's style works best when:
1. There is a clear goal to be achieved.
2. There is a clear vision of what needs to happen to achieve that goal.
3. There is a substantial financial incentive at the end of that goal.
With SpaceX, you want to make rocket launches cheaper, so you need reusability, and there are a lot of opportunities to monetize space if you can make launches cheaper. Musk knew what he wanted and what needed to happen to get there.
Musk has no idea what he even wants out of Twitter. He never wanted to really own it, and now he has a $1 billion bill every year until he can figure out what that is. That's why he's flailing around, throwing out a bunch of ideas to see if something can be monetized.
The fact of the matter is this: if he's staking his hopes on the One Good Idea that will make Twitter profitable in 6 months, given that they're making $1 billion less each month than they were... that's no gonna happen. That's just not possible.