There are places to write that aren't hollywood, you know? Chances are their BATNA is a bit more developed than "go to doordash".
Writing for American TV and film is a very unique skill set and virtually everywhere you would work is covered by the union. All productions have been shut down.
The same union covers if you're writing in New York or Atlanta. And they're not going to pick up and go to London or Mumbai.
The backup plan really is DoorDash, Uber, etc.
...But creative writing in general doesn't tend to pay the bills on a regular basis without the exact types of residuals the studios are actively trying to take away from them. So they're already used to having to pick up side gigs or even have day jobs to make ends meet.
Writing novels is not a reliable source of income, and fanfic doesn't make money at all.
It's true that TV writers often do have to pick up some kind of work in between seasons, though it depends on the show.
But you also can't make a living writing for TV year-round (each gig is on the order of 2-4 months, from what I've read)...unless you earn enough residuals.
So...wait, that would mean that without solid residual schedules, TV writers...just can't make a living??!
And those residuals are exactly what the studios are trying desperately to take away from writers.