The confusion probably derives from people not understanding what the purity routes are supposed to be.
So when we say we're at the middle of chapter 4 and there are 6 chapters (and that one should be the shorter one), one would assume we're way past 50%.
However, the purity routes are individual routes for each and every character that come after chapter 6 (the dark route). Even if it's one purity route per month, we're talking years here since, again, it's one per character.
The counterpoint would be that if you're interested in, say, Chika's ending, and that's the first purity route we get, then you can close the game by then and we're certainly closer to that (the first ending) than we are to the last purity route, or the true ending. By a matter of years. So we should be getting some closure sooner than the actual final closure of the true end.