I got into it with someone I worked with [who made exactly as much as me.] Asked what would someone buy with $5 billion that they couldn't get with $1 billion. He couldn't come up with something, but was still going to defend someone else's right to have it.
The only issue I can think of is that when people reach that billion, they'll just close up shop cause why continue if your revenue is limited to your spending habits. I'd want the answer to that question to be "out of the goodness of my heart" or "to help people" but I sadly don't think a lot of them would do so
Your commnet reminds be about a story I heard. A American guy graduated college and then volunteered to go out and do good in the world. After a year he checked in with some of his fellow grads and saw how much they were making. He did some calculations and decided that he should come back to the USA and get a job. He got a great job with a big salary, and by living frugally he was able to donate enough to support five aide workers.