Are MMOs killing the single-player game?

Listening to the Penny Arcade podcast, Downloadable Content, as I am wont to do when playing something, the topic of discussion turned to how World of Warcraft has ruined Mike for playing single-player-only games because no matter what treasure is received, or secret unlocked, it's an offering on display only for himself; it cannot be shared, and therefore lacks the proper cachet. At least, that was my understanding; I may be mistaken. Check it out yourself.

Regardless, the concept is sound. The rarity of your epic loot is rendered almost meaningless in a single-player venue. And perhaps the heady thrill of true PvP combat makes solo play less appealing. Being a Soloist myself, this is not too much of an issue to me, but what do you all think? Do single-player games have to work harder, innovate more, to deserve your dollar? Is the future of gaming, as Raph Koster seems to think, MMOs?