When looking at long term investment you should try to stick with alts that are already doing well and staying in top 50. Anything else you will be increasing your risk but also increasing your risk to reward ratio. It depends on what your goal is what you consider long term investment. For someone long term could mean a year but for others it could mean five years. Are you able to hold your coin for years or do you just want to hold for less than one year?
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.