I really think this is a case of confirmation bias. We see Google approaching this dominating mass that's impossible to stop, so we begin looking for ways to tear down big brother and look for holes. So, we notice every spam entry.
I work in search and I can't recollect a time EVER having been frustrated with what Google gave me - and having to go to another engine. And I live, eat, & breathe the thing.
I understand I have a notoriously small sample (as compared to the entire world of search - and I generally search for terms that have been optimized by similar search professionals), but the idea is that really, 10 good search results is pretty much impossible - what matters is that there is always three-four results on every page that are relevant. And without fail, Google continues to deliver this - at least for me.
This process is what occurs for every monopoly. We hate the Yankees, Microsoft, whatever. It's natural to want to bring down something so dominating. But for me, Google is doing an absolutely incredible job.
The only potential I see for something like DDG unseating them (or even gaining legitimate market share) is if Google loses the ability to pivot. If this occurs, the dilution of the algorithm is possible, which could turn their product to crap.
BUT, given their history of 200+ algorithm changes per year, they have shown that not to be the case. Because of this, I imagine Google will be with us - & strongly with us - for the entirety of our lifetimes.