This is the fundamental problem with macro economics. there is no way to isolate variables. So obviously saying it is 100% attributable to Reagan is probably

This is the fundamental problem with macro economics. there is no way to isolate variables. So obviously saying it is 100% attributable to Reagan is probably not true. but also (to take some of your points) global competition would effect firms profit margin and theoretically depress income growth. but it does not necessarily explain why income growth shifted away from the middle classes. the issue is more about growth distribution than the presence of growth.