This is excellent!!! Although the back-scratching thing is really what makes this system works best, it should only be a natural thing, as you've described. Seeing a thoughtful comment - even if brief - and feeling inspired to check out the work of the person who has read yours.
But this misery of telling people to check out your stuff - it's awful. I wrote about this recently so we are definitely on the same page!
I have to say, though, there are times I've read a piece and think it's great but I'm pressed for time and have a million tabs open for writers I want to read because they left great comments and I still want to write my own piece and (fill in the blank but time is precious and I have other obligations). So I might only leave a short note like "this was brilliant, thank you for sharing" - but it doesn't mean I didn't actually read. It means I'm exhausted and enjoyed it but don't have the brain power or the minutes or whatever to expand.
Sadly though, there are too many who do write those kinds of comments in the way you've suggested.
Thanks for writing this. I'd like to think pieces like this one and the one I wrote on the same general topic will change the behaviour of the pinheads who need to see them, but I rather think they won't.