That seems to be the main talking point on OH aka sending him down to AAA will do no good because he has already proven he can hit AAA pitchers. Some also contend that AAA pitching is really weak. I don't really know what the answer is frankly, because I don't think sending him up against major league pitching only to see him fail is all that beneficial. At some point you need to instill some confidence.
If a team wants to trade us any good starting MLB with even two years of control, I'd be fine with that. Unless there is some hidden trade stipulation in his contract that I am unaware of. Like for instance if the Diamondbacks wanted to trade us Zac Gallen (they would never do that, just picking a random pitcher) for Holliday straight up, I'd do that.
I agree at some point we need to trade some of these hitting prospects for some pitching, it makes no sense to stockpile all this talent if we can't play them all. Mayo, Stowers, Norby, and Kjerstad are just rotting away in AAA. At some point Bradfield is gonna deserve a shot too. We need pitching, we don't even have that many quality arms in the farm system. All our top prospects are hitters.