This may be less of a concern in stabler, larger coins like btc and eth. I'm still not sure I get why staying longer yields any sort of edge, though. If my expected return over some small period is 0, adding a bunch of those small periods together does not yield a positive expected return.
One way of looking at it is to say that traditional currencies are constantly devaluing as their supply increases. A counter argument there might be that the supply of cryptocurrencies is increasing generally. This could be tested if BTC or ETH are ever unseated by another blockchain.
The psychological conditions could change if every new cryptocurrency was viewed as a potential heir to the throne. "Digital gold" could be debunked in the eyes of the masses.
There are many asymmetries between retail investors in crypto, and hedge fund investors. It is difficult to predict which hedge fund will win, but it is predictable that individual investors will lose on average over time (although some individuals will win).