Eh, it's true that Google's incentives aren't really aligned with bookmarking tools, but the magnitude of the effect is much less than you're thinking of. Basically 100% of Google's revenues come from commercial-seeking queries, cases where the user has a purchasing intent before they came to the search engine. All of the non-commercial uses - coding, random facts, research, education, navigational queries, etc - are just loss-leaders so that your fingers automatically reach for Google when you're searching for lawyers, contractors, home-improvement fixtures, apps, furniture, flowers, restaurants, consumer electronics, etc. Google (of the 2010s certainly, but also true now) considered Amazon to be its primary competition, not Yahoo or del.icio.us.

They put precisely zero effort into killing bookmarks. I know because I was there, and probably would've been tasked with doing it. Well, that and because bookmarks still exist and I can still use them. They also put zero effort into improving bookmarks or making them easier for you to use, but it's death by inaction and consumer choice, not by anything Google did.