Because when new bull run will arrive there will be new shitcoins and your shitcoins will end dead or delisted from exchanges which can lead to 100% loss.History is always repeating.
5,5 years old chart:

Surprisingly some coins from this chart are up from then but volume on exchanges is low,so holders of these coins are truly bagholders.
At the year 2018, if you holding altcoins and wait for the end of the year, I think you will earn a huge profit because of the market has to bull run recent days and not any sign to stop the uptrend. So that I think the holding of shitcoins will be profit in the long-term, not bad as you said.