And yet, stock prices can swing very rapidly as new info comes out, or as circumstances change. Almost like, despite everyone's best effort, they're at best a very crude approximation of future value.

Current stock prices are our best predictor of future stock prices. You are right that they are still a predictor with a lot of variance.

Stock prices should change as new information comes in, yes. I don't see any contradiction here with what I wrote earlier.

You don't see the contradiction with

> The share price today already reflects the long term outlook for the stock.

as an excuse for artificially inflating the stock price? In a context where we're specifically expecting new-but-honestly-predictable information to push it down later?

There can only be predictable news-that-pushes-the-shareprice-lower, if all the people who trade the stock are idiots.

You just need eg a few short-sellers who anticipate those predictable news. They can make a killing by selling the stock at the higher price now and buying it back at the lower price later.

If enough people take this opportunity to make easy money, that brings the share price down today as if the news had already dropped.

So, how do you want to 'artificially' inflate the stock price? Especially how do you want to do it in a way that doesn't require everyone in the market to be idiots only?

If this story was true, we would never see large swings in stock price, because they would already be priced in. But we do. Stock price is simply not an accurate enough predictor to create the incentives we want.

(The most obvious reason this is nonsense is that sometimes the future fall is only predictable to people inside the company. Short sellers don't help here. We need another incentive for those people to do the right thing.)