I believe a major reason for the widely observed decline in Google's SERP quality is that SEOs got too good at gaming the system. The Panda and Penguin updates

I believe a major reason for the widely observed decline in Google's SERP quality is that SEOs got too good at gaming the system. The Panda and Penguin updates a few years ago kicked off a trend of prioritizing content from highly authoritative domains (e.g. big brands) even when that content is less relevant to your search query. Google has been doubling down on this direction progressively--their justification being that people game the long tail by producing a lot of low quality, highly niched content. There may be no real solution to this within the context of Google's PageRank-inspired algorithm and this may be an opportunity for disruption.