For me I sell all the Altcoins that I mine basically for bitcoins because it's gaining value than all cryptocurrencies and it's also part of my dream to raise at least a total 21 BTC and save for the future.
Why you need to sell all your altcoins for bitcoins if you can hold them both? Just sell the half of all your altcoins and hold both bitcoin and altcoin so you will not miss the opportunity of getting rich. Now a days, both altcoin and bitcoin is rising so there is no valid reason for leaving altcoins because we can get benefits from both of them.