I agree with your premise in general; I do not agree with it in this case. Milone is simply not a good enough pitcher that he should justify trading the long-term potential of Owens or Estes (or even two of Devito and Vines), even if that potential is extremely remote. And this is a belief I held well before Milone's disastrous first start last night.
Also, the fact that those four guys are not in the sixty-player pool is irrelevant—especially in the case of Estes and Owens, who are extremely young. There's a reason Makhi Backstrom isn't on the sixty, and Bryce Ball is—and it's all about age and advancement. Honestly, it's a huge testament to how strongly they feel about Michael Harris that he is on the sixty—and his presence/potential/advancement is a big reason why I would've been so comfortable trading Waters for somebody like Clevinger.
I think you'll find that the PTBNLs going to the Orioles are guys like Greyson Jenitsa or Thomas Burrows—players not on the sixty, closer to the majors, who might work out, but whose futures are likely marginal—and not guys like Estes and Owens, who have big-time potential, were payed big bonuses, but are big question-marks at this point.