I suspect this is exactly it—a lot of what made 2005-era-Google good wasn't necessarily Google's own doing. It was that SEO people hadn't yet fully figured out

I suspect this is exactly it—a lot of what made 2005-era-Google good wasn't necessarily Google's own doing. It was that SEO people hadn't yet fully figured out how to game the system yet. If you took an exact copy of Google circa-2005 and had it crawl today's web, you'd probably get mostly "SEO optimized" irrelevant blogspam.