Lucky for both of us economics is a philosophy instead of a science, and condescending attitudes can go both ways. :^)

Suppose as you say, all capital grows uniformly (which it doesn't, but that's another issue) at +x% annually. Now suppose that certain demographics experience -(x+n)% of capital transfer, in the form of housing, healthcare, and other expenses. It doesn't matter how fast the "pie" grows as along as capital transfer away from the middle class outstrips the rate of capital growth. You're cherrypicking hypotheticals here.