I tend to think this is true. I also tend to think that enough people trade based on technical trends that it should be possible to make money by predicting their behavior.
But it may not be possible for mere mortals to do this — perhaps all the profit is sucked up by HFTs with supercomputers.
A lot of people trade based on technical indicators and there's plenty of simple strategies based around moving averages that aren't complex.
In my hometown of Chicago, a lot of people used to be employed in the pits at the CBO market making. (Basically being around to take the opposite end of a trade and making small profits by selling/buying at better prices.) But those jobs were eliminated by HFTs. HFTs absolutely sucked up a ton of profits. All the traders I spoke to who had traded in the 80's, 90's and 00's said that trading got much much harder from the years 2005-2010.
Retail traders still have the ability to take speculative risk and buy/hold. It just takes a lot of time.
But it may not be possible for mere mortals to do this — perhaps all the profit is sucked up by HFTs with supercomputers.