One of my favorite DT songs ever, Voices, is and feels epic and is "barely" 10 minutes long, it's full of twists and changes of mood.
Imagine that for whatever reason that song "had" to be 15 minutes long at least, it would have been a bloated disaster. Well, never say never, I guess you can attach somewhere another section that doesn't feel forced, but..... those 10 minutes are so full and there is no need for more.