Sure, but the money Bill Gates spends on medical treatment takes resources away from other things. Even if supply is elastic enough to adjust, it's unlikely to adjust immediately, and the adjustment will still reduce resources that would have been consumed elsewhere.

Money is a claim on future productivity. Spending it causes people in the world to adjust what they're doing to satisfy what you're asking for. Most of our claims aren't very large though, so this isn't particularly noticeable except in aggregate.

Yes, if you have to treat everyone with Alzheimer's or cancer, that could be expensive. On their the hand, once you develop inexpensive cures, it's not such a burden.