In the last 20 to 30ish years, there's been a rise in anti-vax rhetoric, including massive campaigns specifically against the MMR vaccine.

What makes you think it hasn't increased?

What do you mean, the same group of people? You take the vaccine once, usually when you are a child. So if you got it for your kid, and they decided you are an archaic, brainwashed fool and didn't get it for theirs, there you go.

The Amish and groups like them aren't themselves the main issue. As they by default tend to stay amongst themselves in their communities. Which limits a lot of exposure both with stuff going in and coming out of their living spaces/communities. The main cause of this shit are the folks that are either too "woo woo" and think that basically all diseases are actually "curable" with shit like crystals and the energies of "mother Gaia." Along with their more conservative counterparts in the "God, Jesus, and prayer" side of things. In addition to the overlapping groups that think that all vaccinations are "government mind and body control" shit.

So if more and more people are joining the anti-science side of things and refuse to see how we literally haven't had to deal with these terrible diseases because of vaccinations. Then it means said diseases are going to start actually spreading more and more again. Kind of like how people that didn't live through hard times that lead to aid programs and protections tend to see those things as "not needed at all."

Of course it doesn't help matters that the sins of the past have made very real reasons for different minority groups to absolutely distrust vaccines due to shit like the Tuskegee and Nazi "experiments." Once trust is lost and very real shit like those things are proven, then I am not sure at what point it is even possible to start over. Unfortunately these groups get lumped in with the medical versions of flat earthers that are completely anti-science and proud of it.

And that is all just the human sides of things. The actual diseases themselves evolve the more they are able to spread. Which literally means that shit we had dealt with dramatically now has more and more places and people to spread into. Which at a point also means that people that did take the shots and were good to go are now being punished by these new versions that are taking hold in the anti-science fucks (that also tend to be the same fucks that won't wear fucking masks and try to avoid close contact while fucking sick). If the anti-science assholes that are refusing to take shots over both real and really dumb reasons (muh freedumbs). Then they shouldn't removed about taking basic fucking actions like wearing masks and not claiming that they are somehow the same as the stars and triangles the Nazis forced people to wear. They are selfish and anti-respecting the lives of everyone around them. They just want everyone to yield to their main character-ass whining about "liberty" while oppressing everyone.