How exactly would it be any different without Google / SEO. Parsing of website content to determine topics would be a shit show historically, or ridiculously computation heavy now that LLMs could conceivably do a decent job at classifying content. So Google created a way for sites to tag the kind of content they have. Pretty much any search engine would need the same kind of mechanism.
And content providers are always going to be incentivized to be the top search result, which means targeting search algorithms. That's just the nature of the beast.
If there were multiple SEO implementations, that just means more work to target multiple algorithms. And the content owners with more resources, hundreds of developers, would ultimately win because they can target every algorithm.
I really don't see how Google as a "monopoly" changes these basic fundamentals.