Or the incentive to create quality content has diminished since it's become much harder to monetize?
I think the reason is that Google rewards following it's instructions for mobile, security, etc, and existing quality content is often kept online merely as a donation to the public. Having the incentives of a spammer and following through on every rumored 2% rank improvement is almost a requirement to get near the top results.
I'd go so far as to say that an optimized page is often an indicia of low quality / unoriginal content.