90% taxes on people with only 200% of the median wealth seems awfully excessive. For someone with $16,000 in assets, this would leave them with $1,600 -- just 20% of median wealth. I strongly suspect no tax system in history has ever worked like this.
Not to mention that taxes are collected and spent at the national level, not at the global level. If you are proposing a global government that administers taxes worldwide, that is an interesting idea, but it's not exactly a place to "start" a new tax policy, given that there is no political infrastructure in place to support it and likely couldn't be for decades even if the world made a concerted effort to make it happen.
Not to mention that taxes are collected and spent at the national level, not at the global level. If you are proposing a global government that administers taxes worldwide, that is an interesting idea, but it's not exactly a place to "start" a new tax policy, given that there is no political infrastructure in place to support it and likely couldn't be for decades even if the world made a concerted effort to make it happen.