The point of this back and forth is simply to illustrate just how little you understand the tax system. You have a rudimentary understanding of the concept of "progressive taxation" but no real grasp of how it is actually being applied.
You already agreed that some people use more infrastructure than others. Well, earning the 2nd $100k requires using more of the infrastructure to do it. As they use more of the infrastructure, they are actually increasing their percentage of use as well thus justifying higher rates.
So, imagine that a person uses 1 unit of a 10 unit road to make his first $100k, that's 10% of the road usage. If he uses another 1 unit of road to make a second $100k, he's increased the total road usage to 11 units and his personal usage to 2 units. Now he's using 18% of the road. If he added a 3rd unit, he'd be up to 25% of the road's total usage.
His second unit of road usage means he's responsible for a greater percentage of overall road usage and thus the 2nd $100k can't be taxed at the same rate as the 1st because he's no longer using the same 10% of the road.
Obviously, I'm just making up these numbers. But the point is that making the second $100k requires that there's an increase in the percentage use of the infrastructure. It doesn't remain flat.