Bradley is not good enough. He is best suited for the 6th man role.
I don't think he is at all suited for a 6th man role at all, since he is very strict "SG" and can't really play other positions.
In order to get enough minutes off the bench, I find sixth man guys are usually guys who are versatile enough to play multiple positions. Turner is a perfect example, as his ability to play PG / SG / SF allows him to get starter like minutes despite coming off the bench.
Bradley isn't that type of guy. He's a strict shooting guard with a very focussed skill set as a scorer/shooter and defender. Yet he does those two important things well enough that he can be on the court 25-30 minutes a night, and I think that makes qualifies him for a stating role.
The whole argument that he's not starting calibre is a crock of poop IMHO. You don't have to be a star to be a starter. You just need to do what you do well, and do contribute on a nightly basis in a way that greatly helps your other starters. Bradley brings consistent effort every night on both ends, he contributes on D at a consistently high level, and his offense is good enough for him to be a 3rd scoring option who can light it up from time to time when the first couple of guys are struggling.
I don't see how he is any less starting calibre then somebody like Andrew Bogut, JJ Reddick, Timofey Mosgov/Tristan Thompson, etc. Even the top 5 teams in the NBA had starters who are not stars, but they are still good enough to start on contenders and so they are obviously starting calibre players. Bradley is as good as those guys, so IMHO he's starting calibre too.