People who have some extra amount an ready to take an aggressive risk can invest in the new project or much lesser known coins and if think can make it big in come say 3-5 years or so then yes probably can stay invested or once they achieve some profit can also exit. But for people who do now know and cannot take such risk then better to stay invested with the altcoins and when market falls one should be looking this an opportunity to buy rather than exiting the market.
for long-term investment in new coins seems too big a risk to be faced. planning duration for new project investments that have good development for at least 1-3 years. after that, what happened was the weakening of their markets. some new projects do not have strong support when the market is down.
The new coin is always a risk, we should always choose the old coin which has completed their product, old coins will have higher chances to increase their price along with the Bitcoin price. If we are looking for long term investment, we should leave them until we make some decent profit. Choosing wise coins will always help us to learn new fundamental analysis about the coins.