For whatever it's worth, Osuna has basically been equal to Miller this year, and may still have more upside than what he's already shown. Miller had what, 2.5 years of control for ~23M? I'm not sure what Osuna makes in his final arbitration year, but it's probably not too different for 3.5 years of control. Plus, Washington doesn't need to add payroll this year to get him, which may allow them to add someone else at the deadline as well.
Frazier was in AAA when he was traded (although just promoted), which makes him a little less risky, and Sheffield was a top 100 prospect as well. As good a prospect as Robles is, he is in A+, and probably only has average power potential, although that might play up a bit due to great hand eye.
I can certainly understand why the Nats wouldn't want to do the trade, but my point was that I think it's a reasonable and fair one to make, for both teams considering where they are and what their needs are.