Altcoins provide a powerful incentive to undermine Bitcoin's network.
Consider a situation where Bitcoin's price declines to, or stagnates at, a level that is no longer considered profitable by the miners or pools with the most hashing power. Suppose further that one of these miners/pools has enough power to sustain a prolonged 51% attack, or launch short but repeated attacks, sufficient to irreparably damage confidence in Bitcoin. This would result in immediate and intense competition among the most popular alts for Bitcoin's throne.
If the deviant miner/pool had invested in one or more of these alts before the attack, they stand to make a fortune afterwards. Further, if enough coins were purchased, the miner/pool could even control the early stages of competition with buy and sell walls. Such control could be used to maximize profits but is unnecessary, as the alts would rise in value without manipulative pressures while they are filling the void left by Bitcoin.
There is a fortune to be made through destroying Bitcoin.