I mean same deal is going on in Alexandra right now. Several updates in and just got back around to visiting your apartment and seeing the Submissive Neighbor girl again.
I think he is a great writer, but if he strays too far from the "main" routes he gets board and doesn't know how to advance the side-routes.
In LWM, Mary and MC's relationship was great. That is the canon route, he never bothered to make real routes of other characters although he seemingly planned to. Telling Mary's mom about MC's friend was the big one.
In Alex, as you say the submissive girl vanished from the game for a long time. The gun girl too. The only semi-regular side-girl is the blonde street girl.
In Dana, I guess technically the routes were completed, but a lot of the side-content never god added. The lunch content, the weekend dates etc.
In IR, he openly admits he is board with it. That game became way too complex for him. He only updates that once or so a year because he is just board with it.
Luckily, he doesn't seem to be doing this with Rebirth so far....although the black lady from MC's old apartment has vanished lol...