> company valuations are the discounted value of future returns after all.

Yes, future earnings are worth far less than current earnings - especially in a non-ZIRP world.

Stockholders don't even have that much influence. Because of all the index investing, board votes are often decided by say Vanguard/Blackrock, not mom and pop investors

It is only after stocks suffer severe shocks, does private equity spring into action, and discipline executives via the threat of acquisition & firings.

So its is actually executives that can be shortsighted, not an ultra-long-termist passive investment dominated US investor base.

But an assumption about long term gain can still deliver short term returns. Would shareholders behave the same if long term returns were the only returns available?

Based on your first sentence, it seems that by "returns" you mean increases in stock price. Given that, I don't think there's a clear dividing line, since investors are always forward-looking.

I could see only long-term earnings being available but that's something quite different. Shareholders are often willing to put up with that situation, as long as they believe the long-term story (and see that other investors believe it).

A situation that could make short-term share price increases unlikely is if the stock is just way overvalued for its likely earnings in any reasonable timeframe, in which case shareholders will happily take profits rather than try to make the company change anything.

Combining growth momentum from a past net-value-positive offering with revenue from a net-value-negative product change to project a future that will disappear when your users find your first competitor is an example of that strategy playing out that is difficult for shareholders to catch. Another is laying off and mistreating your employees while claiming that you're going to stay on the cutting edge.

It's not in the interest of shareholders to buy into tactics that pump up share price at the end of a quarter but we're talking about an information disadvantage; the seller is rearranging their books with respect to intangibles to deceive the buyer.