Unobviously Obvious
Google has a very weak incentive to “support” content of quality. Put another way, Google’s incentive to “support” content creators diminishes in quality.
Think about this intuitively: the more crap there is, the more stuff you have to wade through – the happier Google is (at least in the short run).
Let me put this even more succinctly. Google doesn’t care about absolute levels of quality – it only cares about relative levels of quality. And the more media it indexes, the stronger this dilution of incentives gets.