I am not the most well read on Marx, but from what I could understand from his discussion on constant and variable capital, the former still requires human work for creating, operating and maintaining it.

Machinery would only be deployed if it was cheaper than the human workers executing the same tasks, and its adoption would lead to layoffs, thus to the reserve army of labor, humans made "cheaper" by competing against the machines.

I am not acquainted enough with the TRPF to comment on it, but my assumption would be that once labor is made cheap enough the capitalists would not have any incentive to keep investing in automation, thus restarting the cycle. I have a vague feeling that the TRPF also mentions the impact on profits of a much impoverished consumer base (the laid off workers) , but I defer to any comrade who can correct my understanding