Mujaki no Rakuen is ending in just two more chapters. Nothing has really been resolved in the story so far, so clearly this series was cancelled, not developed to its full ending point. It's frustrating how often manga just suddenly end compared to other entertainment media.
Hopefully Uran can somehow make the ending work in the next two chapters such that everyone is satisfied.
Sore ga Seiyuu is supposedly coming to an end at Comiket this August. Fairy Tail is ending next week. (With still no announcement for a third anime season, this is really getting on my nerves.)
Even without these three series, there's still 45 more good manga being made. This doesn't mark the end of the manga industry. And when you take into account the many finished series that haven't been translated, like Nichijou and Angel Beats, the number rises even further.
I do wonder how many more series can end without any new good ones being made before we have to start asking that question though.