the sex arc in Tadokoro killed the series so hard that Tatsubon concluded it's best to start from scratch
I thought that arc was fine, but the author didn't really build any plotlines extending past it.
To be fair though, that's one of the many ways to end a romance story. Some end at a confession (the worst!), some end at a kiss, and some end at high school graduation. I'm always an advocate of "the more, the better" but ending on sex is at least better than most of the alternatives.
Also, just a guess, but knowing rich people tropes in Japanese stories, I wonder if Sakurako had to go study overseas or something.