There's lovely little essay talking about how difficult it is to believe that financial markets are too short term in their outlook. To do so demands that said markets are entirely inefficient in their processing of information. And as this is something that no one but would be commissars still believes then it isn't really possible to insist that markets are short term in their outlook. Here's a part of said essay:

Basically, if capital markets price things well (with few ex ante errors, or put differently, the market is close to “efficient”) then maximizing shareholder value is a very good idea. Believing that markets make common and giant predictable errors is the only legitimate beef one can have with maximizing shareholder value, and it’s absolutely fair to debate this tenet.

But instead of confining the debate to this central point, or even realizing that this is the central point, critics attack shareholder value for many ancillary reasons. For instance, they laugh off the concept as vacuous, the absence of a strategy. They attack share‑based and particularly options‑based compensation. They attack markets and managers for being too “short-term."

The obvious point is that if markets are anywhere near efficient (and just about everybody agrees with the weak version and some more with the semi-strong) in the processing of information then the current market valuation is the value of that company from now into the indefinite future. And, given that we are measuring that flow of funds from that company off into that indefinite future then how on earth can this be short term thinking?

Sure, if you are a would be commissar then you can argue that markets aren't efficient at processing information. At which point you're going to have to explain the difference in food supply in London in 1990 and in Moscow in 1990. When that famous question got asked, "Who is in charge of the bread supply for London?".

Quite, markets are, observably, somewhat efficient at processing information. Thus they are forward looking and as such cannot possibly be short term. QED.