If SpaceX weren't there, they'd be working for someone else or themselves - he needs them more than they need him.
That's not the reality of the situation though. SpaceX's current staff isn't the last cohort of people who are willing to work for musk, and therefore are hugely responsible for the company and it's output. They're but cogs amongst the machine that Musk has built. Cogs are replaceable. Cogs don't function properly if not properly utilized by the engineer.
Musk is the engineer. He's the only person at SpaceX, or Tesla, who is actually irreplaceable. I know people rant and rave about "people aren't replaceable cogs" but they truly are, and that's good. If this wasn't the case for 99.9% of the world, society wouldn't function that well. I'm a cog, and I know that. It isn't shameful and shouldn't be viewed as such.
Elon Musk is not a god and his billion dollar ideas would be worthless were it not for thousands of people who have worked tirelessly because they believe in a common goal. SpaceX is nothing without the labor of others - they are the hand that feeds him, not the other way around.
I don't think people who are doing literal rocket science at SpaceX would "go back to designing widgets" if SpaceX folded tomorrow. This implies that they would somehow not be doing meaningful work without Musk's money?
Labor means nothing if not done for an intelligent purpose. For S0aceX employees, Elon Musk is the source of that intelligent purpose. Of course labor beyond what one man can supply will be necessary for any worthwhile pursuit. What matters is not the labor, but the source of the intelligent purpose that gives the labor a common goal and guides it towards it.