Well, predictions aren't useless in a sense that they might affect the prices. For instance, with many positive predictions regarding Bitcoin there's a chance many users will buy out of FOMO which, in turn, would lead the prediction on the way to being self-fulfilling.
We could make that assumption, and it's definitely not impossible. But think about it, we've had a bajillion bullish predictions in the bull market of 2017 that's being made by big names in the industry, mostly even at the upwards of $50k. And a random dude on some forums would make an effect? Nah.