I think your theories are right on. Lots of people still read, but very few comment. And that's precisely because most internet usage these days is mobile, and clicking "like" buttons is easier than composing a thoughtful reply. Even for me, a heavy LJ user, I don't even try to reply to to things on my phone. (The iOS app is just as terrible as the Android app is said to be, I can confirm).

But as you say, I am still here, and seemingly incapable of not posting. Most of my thoughts are longer than a few hundred characters. I always was an overachiever.