I have not done the rules check, but I assume that, if he clears waivers, he can be outrighted to the minors as someone with less than 6 years service time. I don't know if he can decline the assignment (there's some sort of rule based on when you were first called up).
Also, in terms of the luxury tax threshold, he's an MLB minimum guy, I believe. I don't think his MLB minimum is guaranteed, and, if we replace him, I don't think we take a hit. Research project, or maybe Blue911 can chime in.
In any event, if we have a healthy Goodwin, a Stevenson with options, a Robles recovered from injury, there's really not much of a need for Sierra. Certainly not more than Mark Reynolds / Adams / Difo / Kendrick when RZ, Eaton, and Murphy are back. We aren't squeezed before then and can keep a defensive OF like Stevenson / Goodwin.
1) Sierra had barely one year of MLB service time going into this season. But you can only be outrighted once without consent - which Sierra has been. So he can't be sent down without his consent. If he wanted, he could become a free agent.
2) He's on the minimum salary, so there's no reason a team that wanted him would let him pass through waivers. They'd save only the nominal waiver claim fee by doing so. The only time you see guys get through irrevocable (DFA) waivers when someone actually wants them is when the new team wants him but thinks they can get him cheap while leaving the old club on the hook for his salary. Doesn't apply here.
3) Sierra's not very good, isn't young, and doesn't play a premium position. Hell, before this season he hadn't played in the majors since 2014, and nothing he's done this season shows that 3 full seasons of teams passing on adding him to major league rosters was in any way a poor decision. I'd venture a guess that almost every team has at least one better hitter who can play corner OF sitting in AAA. It's very unlikely he'd be claimed.
4) For the reason above, his agent would be crazy to have him decline the assignment - unless he knows he's now finished in MLB and can get him a decent deal in Japan, Korea, or Mexico, all of which are good leagues for AAAA hitters and pay better than the minors.
5) There's no reason to fret if he declines the assignment and goes to Korea because, well, he's just not a productive MLB player. Stevenson can at least play good defense, and he's already in the system. Your last point, in other words, is spot-on.
It's a remarkable fact that Moises Sierra has spent any time at all on the Nats' roster this season considering that the worry before the year was too many outfielders, not too few. The fact that there are three outfielders who are clearly better than he is on the DL for long stretches (counting Robles) is what's led to this, but it doesn't change the reality that even despite that there are
still three better options in LF. When you're in the boat of being a leftfielder stuck behind thirtysomething backup infielders for playing time at your position, you shouldn't expect to decline an outright assignment and get picked up.
