It might be a bit controversial notion. But when I try to look for extensions say for completion framework there are many available and I used ido and then moved to ivy. I knew helm existed and is much more powerful in terms of feature-set, but never used it.
I read that emacs can report on keyboard usage (I think from xahlee’s posts). Can it be repurposed to monitor which extensions are used often in users setup and they can share that report if they wish to public say via melpa like services. I suppose that melpa downloads can be used as a measure of usage.
Not necessarily; lots of people would test some stuff and than perhaps not use it, or after some time go over to something else and so on. I wouldn’t rely on download stats.
It would be certainly possible to monitor which files are required in your Emacs session, and you could setup a public server somewhere on the Internet where such stats are uploaded, putted together and published for viewing. But what would that matter to you what I or some other Joe are using? Learn a thing and build on it instead of switching and trying. As long as it solves your problems who cares what others are using?