The easiest and most profitable way to earn on crypto-currencies is to buy and store them as long-term investments. Today this type of investment provides an astounding level of income that can not be compared with any other.
I agree with most part of your comment, especially with buying at dip and store coins long term then sell on highs! But on the other hand, I do not agree with being the most promising income generation in comparison to many others. There are many others of which you can gain money way more than in cryptos, let's face it, that is truth.