bitcoin is too popular now a days of crypto currency. and bitcoin popular all around the world. you have got bitcoin you just hold it for few days and wait for growth it. and altcoin not sure when it will be grow up but anytime pump bitcoin. altcoin is too risky. you can hold bitcoin one day it would be grow up.
I agree with you that Bitcoin is already establish and grows with time but don't forget that there are some altcoins that are growing faster than Bitcoin. Example is Tether and Verge. An investment in these coins would have yielded fortune by now. Personally I like diversifying.