IW and Miami don't need to be in the summer; those locations are hot year round, they would be unbearable in summer.
Smaller tournaments are just fine as warm-ups for slams; it doesn't need to be a Masters. The tour isn't just about slams
Slams aren't even run by the ATP -- why should the ATP subvert their calendar to cater to them?
Many players overplay and play tournaments that aren't even mandatory, and then complain that they are tired/injured. Can't help you if you're greedy. And even top 10 guys do it. But the ATP needs to do away with the onerous "four 500-level tournament" requirement, or make it easier to fulfill.