The market goes up exponentially over time. It's a mathematical certainty. The reason giving away trading signals is even possible is because the market isn't efficient. But not only that, there are forces behind prices beyond just speculation where signals can be correct over the long term. Also it takes a while to understand if particular signals are good outside of backtesting so there is no definitive 'these signals are perfect and everyone will use them'

In other words, markets have varying scales of information and are certaintly not efficient despite what academia has you believe. Trend following works precisely because there can be underlying reasons for the trend, and traders piling on just reinforces that trend. Where profitability of the signal depends finding robust trend changes as early as possible.