That majority user might never visit the bookmarks again once bookmarked and would likely use Google to search for the website again. So Google has incentive to not improve bookmarking, Though I'm not claiming that's the case.

I do agree with the other comments that if there are some features which solves some real problems[1] even the majority users would use it.

Bookmarks can help solve some new age problems too, I recently found phished website of our Income Tax site as a top result in Google[2] and those using bookmark to visit such sites which we use once a year could have prevented from getting phished.

[1] https://needgap.com/problems/57-i-forget-my-web-bookmarks-qu... (Disclaimer: I run this)

[2] https://twitter.com/heavyinfo/status/1409761416865746956