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.