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.
It's fallacious to assume the contrary. You can transfer billions of dollars of capital in an instant by signing on the dotted line, but it takes decades to create that kind of value. The lumped sum analysis assumption for capital is valid in this case because the rate of capital transfer is many orders of magnitude greater than the rate of capital generation.
Have you considered that there may be many qualified people who want to work on innovative projects like fusion power, but are prevented from doing so because the necessary capital is locked up with the hyper-wealthy? Only by having inside connections and luck can you hope to acquire capital for your projects, but even then you work for years only to concentrate even more capital in the hands of your lender, making the problem worse. This is a legitimate problem holding back innovation.
I don't want backdoors in my guns just like I don't want backdoors in my encryption. It's really that simple.
Also reminder that Marx was an advocate of widespread gun ownership by the working class, much for the same reasons that the founding fathers of the USA were. Solving the underlying economic issues causing suicide and violence would make far more sense than covering up the symptoms. At the end of the day it's all about poverty.