The Brewers are in a development stage. Trading prospects for a 3 month rental doesn't make sense. If we were in the Royals situation, with Moose, Cain, Hosmer, etc. potentially leaving, then you go for it.
If he were to continue coming in games where the Brewers are losing, yes he is replaceable. Although I don't believe I'm assuming too much, but I'm assuming CC doesn't want to throw him in the deep end to see if he swims. With continued success and an increase in trust, comes higher leverage outings. I don't expect Arcia to bat 8th for the rest of his career either.
Sure right now there wouldn't be a big dent in the bullpen if he was traded, but again he has performed when his number is called. Please don't tell me you believe Lopez can be as effective as Hader is right now. And no that is not why I said I wouldn't trade him. I said he would be (is) a key piece meaning I wouldn't trade him because of where he is now and what his ceiling is.
I can kind of agree he is not a key piece right now, but that's through no fault of his own. Being a RP is new to him, so CC isn't going to toss him in the 8th of a one run game right away. As the season goes on, I would bet Hader will be pitching in the later innings.
Even if his wildness keeps him from the rotation, he would be (is) such a key piece to the bullpen. If Hader gets traded, I think it needs to be for the Stromans, Fulmers, and deGroms of the world.