I think it's fair to add more levels. It's fair that someone earning $400K should be paying less marginal tax than someone earning $10MM. The first person is still likely spending a significant portion of their income each year on living expenses, while the second person is earning more than can be reasonably spent, so it's just "NumBeRs gO Up!" at that point.

idk, something like 60% tax above 400K, 70% tax above 1MM, 80% tax above $3MM, 85% tax above $10MM.

What are the downsides?

That might make Canadian sports teams fail since nobody will want to take the income hit to work here? Even then, they could just restructure their compensation to be paid over a lifetime, so they get $3MM/yr for 50 years instead of $12MM/yr for 10 years (or whatever).