I don't find holding of altcoins to be dangerous, because among the entire altcoins listed on coinmarketcap very few grew while bitcoin was declining in value. But now as the price of bitcoin has started to increase, most of the altcoins too started to increase. This confirms profit on holding of altcoins.
Holding is not always dangerous but sometime is very dangerous mate. Now again bitcoin and friend is decreasing so if the value again going down, holding is very dangerous. So we need to learn the risk because nothing without risk.