I'm sorry, Mr. Dyack, but your personal dislike of a particular genre and your lack of insight regarding its popularity does not necessarily constitute an inevitable failure of that genre. As far as I can determine, two fundamental facts sustain social games: 1) human beings are social creatures by nature(we like finding and having groups with which we are comfortable communing), and 2) people like getting something for free(of course, the games are not truly free, as they are paid by advertising, which the consumer does pay for indirectly through the price of the products being advertised; however, it's close enough to free as most people don't notice or pay attention to the fact). One could also argue that social games are plenty fun for those that play them without requiring the near zealously religious devotion and dedication(sacrifice) of one's mind, body, and soul to the game as many "hardcore" games may require. MMORPGs are especially guilty of this problem.
If you want to talk unsustainable, look at the AAA-title business model of games. Now THAT is unsustainable at its present rate of churn. Soaring costs to produce a game that suffers a complete lack of quality, causing significant disappointment to the gamer for the $60+ that he spent to purchase the game; this results in the success of the game being a complete gamble with a high probability of diminishing returns for the cost and effort. Increasingly, it seems, companies are often betting their entire livelihood on such games rather than diversifying more to balance the costs against the revenue. In other words, producing a AAA-title is very much like playing the lottery for these companies.
The biggest problem of AAA-titles is that they are just too ambitious, and developers are just trying too hard with them in having to live up to producing such a high-value product every single time. This is something that can be done once in a while, not every time a game is produced. The constant expectation that every game produced should be this biblically epic, multi-faceted opus sung by a choir of Angels from Heaven, with appropriate accompaniment by thunder, lightning, and a voice from the Deep expounding the Truths of Existence, is just not reasonable or consistenly attainable; not to mention the fact people just don't have the money to pay for that sort of thing every time or the time to constantly invest in playing such a thing. You have to take a break from stuff like that and deal with something smaller, simpler, more intimate in nature. Something creative.
ADDENDUM: Here is an NPR interview with social psychologist Dan Ariely:
Exploring the 'Upside of Irrationality' [http://www.npr.org/tablet/#story/?storyId=127352130]
Pay particular attention to the segment where he is talking about compensation versus performance and the nonintuitive result that people actually perform worse when the possibility of pay-off is very great. The reason? Stress. People over stress themselves if the possibility of pay-off becomes exceedingly high. They just try too hard and end up failing. I think this behavior has ties into the hit-or-miss nature of the AAA-title model. (This is not to say that people should not try to be successful or obtain great gains for their efforts, but there is a point of reason, beyond which things break down. All things in moderation and proper form, as always.)