I'm trying to wrap my head around this.

 The only reason to mine for an altcoin is because it provides the opportunity to exchange them for more Bitcoin than mining for Bitcoin directly.

Two forces will act on XYZCoin to eliminate this opportunity for excess profits. First, I will keep producing XYZCoin and selling them in the market, driving down its price.

 At the same time, the additional mining power entering XYZCoin mining will drive up its difficulty, reducing the amount of XYZCoin expected given the hashpower in my mining rig.

So by definition, the value of every altcoin is always going down. Therefore, it is incredibly stupid for anyone holding BC to trade it for any AC.

So, can anyone explain to me, why is it that people trade BC for AC? Why does the AC market even exist at all?