You are right, it preferable to go for a longtime investment because some coin aren't strong enough and beside there is alot of altcoins that are intend to pump and dump so will all need to be careful and research alot about the coin we are going to invest to.
Investment in the long term will always include the best coins in terms of ranking and trading volume, because from that everyone can see that the coins that are most in demand are coins that are suitable to invest in the long term.