Exactly holding up substantial coins will be profitable when you intend to sell but w non profitable coins does otherwise, so a more reason to buy at low rate and sell higher to make reasonable gains. This is what makes the market so reliable to traders and investors of long term. And with this studying the market to know when it's profitable to buy and sell is something else to always reckon to.
By substantial do you mean the coins that is very weak and unreliable which is altcoins? Or you are actually talking about Bitcoin?, however Bitcoin is not a substantial asset but a quality one. Your advise might be considered opposing to the role of investment people are doing here because when everyone is on the long term objective and you suggest different thing that might not help the objective of such investors it is seen to be inappropriate advise, buying and selling is not always the right thing, profit is good but of what cost?, you might have a profit now but doesn't have your capital again tomorrow.