Shares are a factor in production albeit indirectly. Stock markets were alive and well during the era described in Marx's works so his theories are still valid. Capital is more than just raw machinery and factories. Money and shares are abstract capital. If shares aren't a factor in production why does a company usually go bust when its share price crashes? What about the Great Depression? Share prices and speculation played a massive role in the 1930s crash. Marx spent most of Das Kapital differentiating market value and essential value and so it is with shares. A company has an essential value and a market-traded value. How closely these correspond varies. When the two diverge we have a bubble like the one around 2000.
> If shares aren't a factor in production why does a company usually go bust when its share price crashes?
You have causality backwards.
> What about the Great Depression?
Marx didn't predict it. He predicted a very different phenomenon based on the long rate returns to capital leading to periodic crises, which would end in a revolution and after that he gets a little bit vague.
The Great Depression was an asset bubble that burst with no fiscal or monetary mechanisms available to cushion its effects. It was the most spectacular in a long line of crises exactly like it.
> Marx spent most of Das Kapital differentiating market value and essential value and so it is with shares.
Marx made the same error you're making; which is believing that there is a "true" or "essential" value for anything. His economics was based on the economics of the time, which had not yet solved the diamond-water paradox with the subjective theory of value.
Marx predated the marginal revolution, dying two years before Marshall published Principles of Economics.
Marx was trying to make steel from clay. He was a brilliant and original thinker, but his theories didn't pan out. The crises he predicted do not follow the form he described, the revolutions which succeeded were not in the countries he predicted, the inexorable laws of history turned out to be very exorable after all.
His main contribution since the late 19th century has been as an anchor to progressive economic thinking.
