You're confusing means for ends.

What you're focusing on was that the rich never actually ended up paying those 90% marginal tax rates on their income. But the point of those taxes wasn't to raise revenue; it was to force changes in behavior. Them donating to all those big hospitals and universities wasn't a bug, it was a feature. The goal was to force rich people to spend or give away their money instead of hoarding it. Which is ultimately the goal. I ultimately would rather have someone make and spend a billion in a year than make and save a billion in a year. I actually don't mind if the rich have extravagant homes and yachts, as they have to hire a hell of a lot of people to build and maintain those luxuries.

High marginal taxes also encouraged companies to give better pay and bonuses to their workers. That was actually the origin of the large end-of-year bonus corporate America traditionally gave out. If your company is sitting on a large pile of extra profits at the end of the year, rather than paying the high marginal tax rate, it made sense to give the extra cash out as a large Christmas bonus to employees.

What you're talking about wasn't a loophole. It was the entire purpose for having those high rates.