@Exile
This is a manga, it have a linear narrative, not a CYOA book and certainly not a VN .
This is actually whatever the author wants it to be. You know typically in literature when something is written in a particular manner repeatedly by many authors to the point that certain outcomes are expected by readers we call that a "cliche." If the author wants to break out of the traditional format and provide multiple endings, why not? Certainly no one here is owed a linear ending.
Maybe its just Rizu that doesnt work out but at least Uruka had a feeling of being definite, this is just "what if Uruka goes to Australia" but Nariyuki being removed from being a character and into a self-insert just so shippers can get their ending IS a copout
Uruka is definite, they are all definite, all you have to do at this point is read them decide which one you liked better and move on when the manga is finished. As far as I can tell Nariyuki still has his own personality, likes, and dislikes, so I fail to see how he as become a "Self-insert." I certainly know I don't identify with him in any way, as he is about as different from me as can be. Will shippers enjoy that this series has multiple different endings? Maybe? But you know you aren't being forced to participate in any of that right? I know that I'll be able to enjoy the whole of the story and move on once the manga is finished. You don't have to create a headcanon or anything It's really not that big of a deal.
even VN dont give a route to every single girl that have a unique sprite (until the fandisk or special editions, Mayumi in Shuffle as a example) and each route have its own specific to the route narrative that focus on the couple, not some "generalist harem highjinks" that you then pick who wins out of a choice box.
You should play Clannad mate, I guarantee that every single side character that isn't married or getting married has a route. There are also several VN companies that go out of their way to provide routes for side characters even in the main games, Minatosoft in particular are very notorious for doing so.