Hello and greetings Sirthinksalot,
Well, that is where it is ultimately headed.
But as long as we still have one, I'll address that.
The poor can't pay any more, true, but that also includes the lower half of the middle as well as some in the upper half. People tend to spend what they have up till the point that they have everything they want. So even on an upper middle budget, there could be little room for more taxation. Supposed somebody has huge student loans to pay off, and a large mortgage, and becomes impacted with a personal tragedy such as a kid in legal trouble or a draining illness. They could be in a position where a tax hike could cause them to have to sell off the main house or something.
The middle would generally be able to pay more taxes, but if they do, that often comes out of either retirement savings or consumer spending. Both would have a negative effect on the economy. That effect would be largely cumulative because it would involve so many people.
Really, the rich is the only class which can absorb much increased taxation with the least possible negative effect on the economy.