I used to think so but after some more thoughts, don't think it is true. In general, the US equity market has an up trend of ~5%-10% a year, which means statistically both buyers and sellers of equities, on average, can be profitable (ignoring slippages). I am not sure about other instruments, but in general, one can look at a long term trend (e.g., US currency has a down trend for several decades, commodity has a general up trend due to inflation...) and bet successfully, statistically speaking.