None of the roles in Miss Saigon are so difficult that they should require an alternate. Kim is a very difficult part to sing, but it's no more difficult than Evan Hansen and Ben Platt misses very few performances. If the performers are supporting their singing with proper technique, they should not need an alternate. It's not like they're singing Mozart.
That being said, people do get sick and no one wants to see a performer end up causing permanent vocal damage because they thought that they could power through an illness. That being said, if I were performing in a Broadway show, it would take a lot for me to miss a performance. The audience has paid to see me and I, as an artist, should be deeply excited to get to show off my art. Basically, I keep on hearing from friends of mine that it's nearly impossible to see Miss Saigon with all the principles on. It seems like someone is always out and an alternate is on. These performers should be more than capable of performing this show 8 times a week.