Valid strategies don't permanently stop working if they're based on price action as a manifestation of market behaviour. If market behaviour changes of course, such a strategy will either increase or decrease in effectiveness but it can't become invalid: plus the specific market changes will be visible in price charts. Strategies which are not soundly based might well work for a time and fail for a time. Which can make them legitimate choices for some traders - use them when they work, switch to something else when they don't. And either way, blame the market.
How will you define market behavior based on PA (means candlestick patterns!?) Another question: where is it written that certain behavior leads to certain up or down movement? You can see double bottom fails more than it works as a support or vv. About statistical edge: how will you know that your edge is fading?