Google can trivially reduce the amount of spam by using the amount of ads, analytics, affiliate links, etc as a negative ranking signal.

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.

What incentives do they have to promote affiliate links to Amazon ?

It's a bit more complicated than that. Even useful sites in commercially related niches use affiliate links as a source of revenue.

I’m not saying they promote them explicitly. I’m arguing that they should downrank them because it’s a likely indicator of spam - there’s a lot of spam websites out there which are just bullshit listicles or fake review websites whose primary objective is to get you to buy through their affiliated link. I wouldn’t be surprised if there’s software out there that can churn out and update these websites automatically. Google has no incentive to downrank these though because those spam websites also often contain Google analytics or ads.

Search is their main gateway to the advertising dollars. "Google it" is synonymous with any search regardless of the engine being used.

Wouldn’t in-app advertisement be at least on par, probably higher in usage ?

They serve ads regardless how you get to the site you’re viewing.

They serve more ads if they steer you towards the low value SEO website filled with Adsense.

That is precisely my original point.

Google loses if people stop using it. Google has every incentive to downrank them.

Google has incentives to degrade the quality of search results for the benefit of its profitability, up to the extent that is able to without turning away too many users.

I think it's more like that rather than an either/or.

They have competition: if we get to the point where Bing/DuckDuckGo results are clearly better, people will start moving, and that will hurt their revenue badly.

Yes and those results don’t have to be just better. They have to be clearly better as you stated. The likliehood of that happening any time soon is close to zero.

I personally use Bing for 95% of my searches and have done so since it launched. Or re-launched.

I switched precisely because I found that the DuckDuckGo results were indeed better. This was a third attempt though - the first two times it wasn't quite good enough. It improved, and Google got worse.

This makes no sense. Google makes more money when users click on some crappy landing page filled with display ads?