Shareholders' equity comes straight off the balance sheet. If a divestment campaign succeeds in significantly lowering the price of a companies stock, then the market capitalization is reduced, but the shareholders' equity remains the same.
I find the rule-based argument, I do not wish to profit from the (alcohol, tobacco, defense, gambling, fill in the blank) industry, therefore I will not own their shares, sound. (But that doesn't lead to a sexy prospectus that justifies 0.7 - 1.5% expense ratios for active management. Vanguard Social is the only fund I know of that runs a social fund as a broad index, minus a few industries.)
But utilitarian arguments that divestment actually impacts the future is based on scanty evidence. And if you believe the utilitarian argument, it doesn't let you stop at zero ownership. If divestment is doing positive good in the world, you have an imperative obligation to sell short!