Well it may be a general statement, but any method which is a "buy at X time, sell at Y time" will not work over a long time because the market simply does not allow things like this to be profitable. When something becomes so easily exploitable that enough people do it, then it ceases to become an edge and fails. There is no "easy" money gimmick that will work.
Would disagree with your reasoning. How do you know if there are enough people doing such a method so that it will not work any more?
I'll use another line of reasoning -- give me one example of such a strategy that is profitable over a 1-2 year period. It only proves that it can be done, not that it can't, I know.