Personally, even if they don’t admit to it in public and ignoring any reasoning (school/work requiring it), I think a lot of anti-vaxxers will get the vaccine.
Keep in mind the overwhelming majority of anti-vaxxers had vaccines growing up, what they’re doing is refusing to give their children vaccines. That means that even if their kid gets measles or mumps, they’re protected, and it should be noted, with no side effects.
I’m sure a lot of them will refuse to give their kids the vaccine, but I bet a lot of them will get it for themselves.
The problem with the hypothetical is that the vaccine isn’t even ready yet so a lot of people will be skeptical about trying it when we don’t have any long term data.
I think a more interesting question is, if we found out that the measles vaccine ‘wears off’ after 50 years and requires a booster, would anti-vaxxer parents go and get it? They know that herd immunity is dropping and more (unvaccinated) kids are getting it, so will they get a booster to make sure they don’t get sick?
I’d love to say that anyone that’s promoted non-vaccination should go to the back of the line when the vaccine comes out, but like the virus, the vaccine doesn’t care about how you feel about it and the more vaccinated people the better.