I love this post! And thank you for writing it! I completely agree with you so I'll add my little bit to it too, if that's okay.
I remember when I first began writing, I used to get like 3 comments per update, maybe it was because I had been so completely inactive at the time but to be honest it's just because writing is what interests me the most.
Why I write is a whole different matter to what motivates me to keep writing. I must admit though as a rule of thumb I do not ask my readers to make however many comments before I update, what I do ask though is to comment as much as possible. Because at the end of the day who doesn't like a bit of appreciation for their work.
Although I don't write with the expectation of praise, that is in the end what keeps us writers motivated towards constant updates. Writing fanfics isn't our occupation, in fact we all do it because we have a certain love for creating stories out of other stories, because we have imaginations that sometimes need to be put down on paper, but that we can do on our own time too.
For example, I write my diary maybe six times a year, when I have time, when I feel that I can sit back and write in it in the post perfect handwriting that I have and so obviously it is the same with fanfics. If I had my way, I would probably update my fics once a month and leave it at that so in order for me to update more frequently I need comments and likes, that extra little kick to save me from pushing it to the back of my mind.
Because let's face it, we all have a life outside of writing, most of us a very hectic one at that and for us to take the time required to actually type up an update is not easy to find. I myself, like you had mentioned have stayed up till maybe 3am at night when I have to wake up at 6am in order to finish an update. So if we feel so genuinely about what we do, then why can't readers give us a 2% fraction of the time we take to write them in order to type up a comment.
As much as I absolutely adore long comments, even a short one will do, a little bit of appreciation as they say, goes a very long way.
I know that infrequent updates are a personal bad habit of mine, but life really does get in the way sometimes, but I do feel that sometimes my updates come so late due to lack of enthusiasm. Maybe not everyone likes the same concepts or styles of writing and maybe we are expecting a bit too much but I'd really encourage people to comment. Putting up an update takes a lot of time and effort and shaping out a story even more. And if the story then keeps your interests and you enjoy it, I'm sure a comment or a like isn't much to ask for.