When you always keep the mined altcoins, you will die one day as the guy with the most altcoins in his wallet.
You are right, alt coins get depreciated over time whereas bitcoin gains its value if the miner hold for long haul.
I say if you can sell them, sell them!
Nobody knows if they will rise or fall in price. But you know the point when you make profit.
But there are few altcoins like litecoins and dogecoins which are stable for years regardless of what the value bitcoin is having. Mining those coins and holding them will definitely fetch higher networth over the time.
Just select what you think is going to be big in the future, there's no challenge in bitcoin as it is already progressive and we can predict the price will increase but unlike in the its early stage that it turns out to be a valuable asset. If you believe in some alts, just participate in the ICO, they have a good price.