I know the story goes on, but this would have been one heck of decent place to end (sort of an open-ended, "and the journey continues"-type end).
I do feel for Subaru-sensei, a bit, but she pretty much already succeeded. Even if it's the "end" for her, she's going on near the top of the game. No shame. Besides it's not like her life's at stake, unlike Shiori.
EDIT: much as I love this manga, there's several reasons I think it could never be animated. First, it spends a lot of the early part of the story putting down mass-produced, nice but forgettable music, which is the exact kind of music that anime, idols, and seiyuus pump out by the truckload every season. It's risky to make a product that basically calls the tastes of your target audience crap. I mean, speaking for myself, I have tons of anime/JPOP music, but I'll be the first to admit that none of it will be noteworthy in 50 years, unlike the legends that this manga is an homage to.
Second, Shiori Experience's music is supposed to be on the level of Nirvana, Hendrix, et al. It's really unlikely that an original composer making music for an anime adaptation could reach that level of genius - if they had that talent, they'd be a legend already. That means that we're going to have to pretend that "JACK In" really is a work of generational genius, where in fact it'll likely be "pretty good" at best. And I think that would be a disservice to the music that this manga is elevating.