100% the way it should have ended, from the romance perspective. I disagree that this was a "twist" ending like many people think, as it was being foreshadowed from VERY early in the story. Chihiro is the only one that liked him for his natural characteristics, she admitted to this much earlier in the manga (that she had been in love with him even before Elsie showed up).
This last story arc was a mess though, hard to get through. Not very interesting. It got too far away from its romcom characteristics which is what drew people in - I'm not surprised that they didn't bother adapting the last arc into an anime. Wakaki also could have done a lot more with Elsie. He really screwed her over a lot by basically ignoring her for the entire manga (he even makes self-referencing jokes about it) then barely coloring in her backstory at all in just the last couple of chapters - but at least he gave her a cute ending with her wish-fulfillment about having a real family, so good I guess? Seems like he just got tired of drawing this manga and wanted to move on.
Still, Chihiro was objectively best girl and I will fight anyone who disagrees. It's rare for best girl to "win" in romcoms, it's usually some shitty cardboard personality character that appeals to the worst aspects of the JP fanbase (sorry but "painfully shy childhood friend" is one of those).
I also disagree that MC didn't grow at all - it just wasn't presented well. His growth is shown mostly through his relationship with Chihiro and his regret in how he dealt with her (she was the only girl that he experienced any regret over, and it was shown from very early on that she was the only girl that could ever get under his skin and make him depressed). He goes back to her because he realizes that a real relationship (one not based on forced affection, like the others were) has meaning in the "real", as he calls it.