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.
I think by substantial he means a valuable coin so my guess is that he is referring to Bitcoin. He is not just opposing what people are doing here but he is opposing the ideal investment plan for all investors.
A Bitcoin investor shouldn't be thinking of or bothered about selling them because long term holding is the real deal, instead how to accumulate more should be the priority. You should understand that selling will make you miss out on future gains when the price increases since Bitcoin value is known to increase over time and also that means buying back will cost you more.