An efficient market kills profit, essentially by definition. Whether public securities are an efficient market is a whole different question.
In general, a profitable strategy needs enough capital deployed into it, or into counter strategies, to actually shave away the alpha. Unless these strategies get massive exposure with millions or even billions of dollars at play, they'd avoid this fate. Even still, a few years of beating the market can be all it takes. Remember: "beating the market" isn't important, getting a good return is. If I beat the market by 10,000% in 1 year, then underperform by 2% for 30 years, I'm still very rich.
Also food for thought: the most popular strategy in the world - "buy and hold the top 500 public companies ranked by revenue and weighted by market cap" is still profitable and has "alpha" over the previous benchmark (treasury bonds).
Perhaps another consideration is that strategies based on real economic indicators and underlying physical realities (like increasing corporate profits and low rates) have enough momentum to maintain profits even with copycats.
