Huh, wow. Did the anime for Tonikaku and SLeepy Princess really do noting for the manga sales? I thought there would be at least some notable boost, but that doesn't seem to be the case.
I'll play Devil's Advocate here. Following is just my opinion:
Yeah, the entire point of the series was the ecchi. However, having actual romantic progress and ecchi isn't exactly mutually exclusive - see High School DxD for example. IMO, the problem with romcoms like these is that the initial premise tends to wear itself thin fairly quickly and the characters aren't usually compelling enough to pry people's attention away from the romance. (Then again, shipping is a very powerful motivator for people to watch a series in general.) Thus, the only developments people really care about are the romantic ones - it is a romcom, after all, but since most romcoms don't usually have their leads become a couple until they end, the manga just ends up feeling as if it goes nowhere until that point. It's worse with Amano Megumi since from a chapter count standpoint, Amano Megumi is long. Longer than Nisekoi, Senryuu Shoujo, Quintuplets, and We Never Learn - all of which IMO have premises that last longer than Amano. And last I remember, there have been moments where it baits the idea of the main leads becoming a couple before quickly backtracking.
Meanwhile, pairing people up midway through a series after some buildup instead of at the end, I find, makes said series more memorable - Kaguya-sama, Jitsu Wa, and even Chivalry of a Failed Knight will still be talked about for that reason. People want to see a couple they put stock in actually be a couple, do couple things, and take on life together (there's a reason as to why Clannad: After Story is regarded as one of the best romance anime ever, after all, while you don't hear many people talk about the first season). Now, I'm not saying that Amano Megumi should be like Clannad, but...people are kind of tired of the same old romcom, I guess. You can have Amano and the MC as a couple and still be full of openings, ya know? People would enjoy it more. Hell, you can even come up with interesting scenariosthat take advantage of that.
TL;DR: There isn't much to be invested in besides the fanservice and the romance, but the fanservice doesn't exactly keep people reading in the long term, so people look at the romance - and get frustrated because the romance barely progresses over long periods of time.
This si a bit sloppy, but it's really late at night here. Just wanted to say my piece.
