There should be a rule of clue flight somewhere. But it's a highly established pattern: great minds seek out a high s/n forum, it becomes established. Over time the s/n ratio falls. How long that is varies greatly (this is a problem that greatly pre-dates the Internet), and can be affected by various exclusion mechanisms (moderation, subscription, rate-limiting, etc.).

Eventually, though, most forums fall prey to low-value contributors. There may still be gems (and often there are more and bigger/brighter gems even in the age of noise than in the heralded golden age, there's just a heck of a lot more noise), but the perception is of spoiled ground.