The way I see it, they have 3 options:

1. "Ice the kicker": Refuse to negotiate for several more months at least, and hope that the WGA comes back begging. I think this is most likely going to be the response of the WGA doesn't agree to the terms, as they have too many projects riding on union members.

2. Import writers and actors from other guilds. I wouldn't expect them to do this unless we get to 2024 and there's no deal. There are plenty of fantastic writers and actors all across the globe who could easily make great stuff for Hollywood (IMHO they could do it better than the Americans but I am perhaps a bit biased, and somewhat uncultured as well). Risky because Hollywood would need to either reboot or place on hold existing IPs, meaning no more direct sequels to stuff, nor further development on things such as Marvel movies, at least as we know them.

3. Switch exclusively to non-union American workers. This would be a last-ditch effort, I think, as this would cause big-name actors to work for independent American studios such as A24 or Angel Studios, causing them to rise to prominence and most likely overshadowing the major studios over only a handful of years.