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.
which extensions are used often in users setup
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?
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?