mining alt coins and saying they are counterfiet is like trying to trade monopoly money for real money. Only works if someone wants to do the trade
To go on further with this, the reason anything is valuable (money, gold, bitcoins) are because of perceived value. You could tell them that monopoly money could be used as real money, as long as merchants and people in general were willing to buy and sell with it. Bitcoin has the security to enhance the trades, and because people have and already do accept bitcoin, that perceived value is there making Bitcoin worth something.