That's not true in all countries. In France no matter what your wealth is, when you get a long-lasting disease like cancer you get 100% coverage from the public healthcare for that and can be cured in high quality hospital.

Now if you're extremely rich you might be able to get a famous surgeon that middle class doesn't have access to, or get a new treatment only available in foreign countries, but at the end of the day I don't think it's going to have a huge impact on your chances of survival because what you get from healthcare is already really good.