I recall reading a book review in Scientific American about the last great smallpox epidemic, which hit the Northwest Coast Indians in the late 1800’s. The Victoria BC museum has (had?) and exhibit deidcated to it, too.
The author’s thesis was that the natives were no more likely to die than anyone else, white or brown. What did them in was subsistence lifestyle and virulence. In european population, a significant number would be immune from prior exposure. In native circles, nobody was immune and everybody caught the disease within a few days or hours of each other.
The museum exhibit, for example, recounts newspaper stories of finding boatloads of natives who died in the forest. The entire canoe crew became sick on their way home from the trading post, pulled in to shore and died together.
What did them in was that during the worst of the disease, there was nobody to look after them and they died of dehydration and starvation. The book author found accounts where when there was even one person (often a white missionary, or a native who had spent time in white settlements) who had immunity and could properly care for the infected persons, feed them and bring water, the survival rate was often close to that of white people who caught these diseases. (The figure he gave for smallpox was 10% mortality if the victim received proper care).
So there’s no magic special susceptibility if the population has not been vaccinated; it’s more the question of “what if everyone got sick to the point of incapacitated at the same time”?
I assume the reason MMR are not eradicated like smallpox - despite the seriousness, they are nowhere near the level of lethal as smallpox; nor (AFAIK) the devastating disfigurement. I recall pictures of smallpox victims in remote areas of Africa during the last of the eradication drive - some were covered with tiny boils (small pox) which apparently leave scars like the worst acne sufferers.
I had mumps when I was in about grade 7, not long after my brother; the only medical effort I received was to stay home from school for a week with massive throat swelling, and my dad helpfully warning me to let him know if my balls started to swell up. (fortunately, no.) It was uncomfortable, but I’ve had worse cold/flu.
Another good reason for smallpox eradication, was as mentioned above, there was one variety. There was cowpox, a mild variant which is credited for paving the way to vaccination techniques; apparently “milkmaid complexion” was associated someone who caught that from cows and so never got smallpox or the scars associated with it. Compare that with diseases like AIDS which have defied all effort to find a vaccine against it.