This is an exaggeration... but I guess one's definition of success is subjective. But I mean, he is well known to "the scene" but never had any real mainstream success. Honestly, it wouldn't be a huge stretch to see him on The Voice considering they have had some known artists on (some who had already been on major labels). I doubt it would ever happen though.
He is more successful, including financially, which would just cause a shitstorm of "he's already financially set off his career why does he get to be on this show" anyway. And he definitely has a more successful music career than any previous winners, regardless of your bullshit "mainstream matters" attitude. He's definitely not just well-known to the scene. Have you seen any Andrew McMahon/JM/Soco shows recently? "The scene" makes up about 10% of the people there, everyone else is college kids and people not part of any "scene." Effortlessly selling out huge tours, not seeming to have a care in the world for where his career has taken him and is heading... yeah, I think he's pretty damn successful for a TV show made for unknowns who have virtually no active career. The only logical excuse they have for Cassadee being on there is because her music career had gone down the drain.
I mean, maybe if the world just forgets about him and he can't sell more than a hundred records and no one goes to his shows anymore, then it would probably make sense.