By that line of reasoning, we may as well eliminate tax brackets entirely. Or any sort of arbitrary limit, really. What makes an 18 year old mature enough to vote, but not a 17 year and 364 days old?
Humans, as all animals, have a built in maturity limit, which is puberty. After completed puberty you are biologically an adult. You can be a completely immature fool though biologically adult, as well as at 18, at 38 or until death. I'd say a majority of people live their whole lives without ever reaching adulthood in a spiritual sense. Go work in the service industry catering to an elderly clientele if you don't believe it.
I appreciate your mathematical approach but what I consider bad is not having $1e9 or $9e8 but ability to pay less taxes per dollar than poorer folks. Bad thing is that kingpins of: mining, slavery and proprietary software are so rich while providing so little value to society/environment.
Examples: mining materials from Earth is important, but the revenues should belong to all who live on the planet being mined. Slavery situation (I mean Nestle corporation and most of Cobalt production) is a pure shame. Proprietary software is a shame as well because it effectively converts users into digital slaves, effectively this is a branch of Mathematics which is forbidden (and obfuscated) to learn.
That's the thing I don't get: the vast majority of people are not rich, and will never be. Why do they fight so bad to help the rich people pay less taxes?