If you're wondering why there's so much belief in a poly ending for this manga, you might want to re-read chapter 8. That's the one where Mei learns two of her teachers are in a poly relationship, and that the three of them met while students and roommates at Mei's school.
It draws some pretty strong parallels between those three and Mei, Sayo and Rinna. Mei even returns from this meeting and tells them both - at the same time - that she loves them, in addition to smiling when she sees a love arrow going from Rinna to Sayo. She then takes both of their hands and sits on the bed with them (again, all together) and the chapter ends with a photo of the three adults when they were at the same age as the main trio.
I get that fans can get unrealistic ideas stuck in their heads and then get disappointed when they don't pan out, but to say that series never teased a poly ending seems incorrect. It may or may not have been the "original" plan, but it clearly was put on the table as an option.
Ultimately, I think if it was planned, it likely was more a casualty of time than cowardice or meddling. The author likely was told the series was cancelled with only a couple of chapters left to wrap it up, and since Mei's relationship with Sayo had been more explored (so far, at least), it "defaulted" to that ending. Notice how Rinna was just kinda...there at the end. It seems like the author had to rush to figure out a halfway happy ending for her after not having one ready, so they went with the "well, she loves everyone and everyone loves her" thing.
I'm certainly disappointed in how it ended, though I guess it could be worse. And I definitely will indulge in a little headcanon and say that Mei and Sayo got together with Rinna now that they're all adults, because it makes me happy and you can't stop me! :P But I'll mostly just put this one in the "I liked it, but it ended too soon" bin and move on.