Isn't this an example of the GP's point?
There's no intrinsic reason for earnings to be directly related to stock price.
Like you said, it's a justification not reasoning.
Would you buy that for $1? What about $1M? Something price in-between? Does the price change if it’s Bill Gates offering? What if it's the US government? Does it change based on your life expectancy, the tax treatment of that $100, expected inflation rates, if a bunch of investors want to buy this contract, or if the payment is “maybe $100, but sometimes more or sometimes less, within a reasonable range”?
There is a calculable and narrow band for the fundamental price of this contract, and it’s intrinsically based on two things: how much I promise to pay back (~return) and how believable it is (~risk).
"Corporate earnings" are equivalent to this monthly payment, just the payments take the shape of dividends, or share buy-backs, or reinvestment which promise even greater corporate earnings.
>... and re-investment is extremely fuzzy and hard to value.
Yes. And the more this is true, the more that "stock" becomes a "stonk".
Yes there absolutely is. If you are the owner of a company you may divert the future earnings to your own bank account, which has a precisely defined value.
Your counterargument may be that minority shareholders never have the power to affect distributions of earnings, but that is also false. Minority shareholders at minimum hold the power of arbitrage between majority shareholders.
You are buying a share in an income stream! You are using your money to buy money! If there is no intrinsic reason for this to have a price that is measured in money, where exactly do you think a price measured in money is appropriate?