Right now SEO it's an arms race between google and every SEO agencies, with a keyword being attached to some company that may have 0 relevance to the user.
Doesn't help, that depending on the query, the first results are ads only superficially related. You can search for the name of a popular brand of a car/hotel/insurance, and the first results are the competitors. If you don't want to be below the fold, you need to pay, even if your website it's exactly what the user it's looking for.
It was never the case. As soon as google started to be popular, people were hired to manipulate its search results. "Natural" SEO was never a thing. You always had to use shady tactics in order to get a good ranking because everybody was doing the same.
In that sense Google's monopoly is actually preferable for marketeers. It would be a lot harder to manipulate search engine results if the public was split among 20 of them.
I think it's more like that rather than an either/or.
I personally use Bing for 95% of my searches and have done so since it launched. Or re-launched.
Instead of targeting SEO per-se (as it's difficult to determine a good website with SEO vs a spam website with SEO), target how they're making money.
They're not doing so exactly because they have no incentive.
It's a bit more complicated than that. Even useful sites in commercially related niches use affiliate links as a source of revenue.
This makes it much harder to be a good search engine.
I have no idea how to quantify how much the decline is because of that.
Google refuses to hire humans to do stuff like this. Their service is terrible. Maps requires free labour of millions of users to get information updated.