Hoskins wasn't good at playing 1B. Didi wasn't good at playing SS. Bohm isn't good at playing 3B. In Harper's case (and Hoskins' too, and even Schwarber's last year) the bat outweighs the poor fielding. Don't get me wrong, I think Harper will ultimately end up moving positions. I just don't think the machinations of this season indicate any urgency on the part of the team to start that process beyond doing it this season because of his throwing arm. 2 years from now, sure.
And realistically there is only one starting job open regardless of what Harper does. If he's the 1B Castellanos is going to stay in RF and Muzziotti, Rojas, Pache and Marsh (all four of whom are still young players) are competing for two spots... but probably just one unless one of those bats truly plays in LF. If Harper is in RF Castellanos is probably the LF with one of those guys on the bench.
Neither Schwarber nor Castellanos are going to play 1B in my opinion. And having a poor defensive 1B, as we already know is not great either. It may be easier to play than OF but you handle a whole lot more balls.
I certainly don't trade Rojas for a rental and am not sure what Muzziotti really gets you even with this current season - he's a piece, not a cornerstone, just like Nick Williams once was. But that is still the area of tradeable depth just like catcher was last year. And I still think we will see someone who is not currently on the team playing LF or 1B next year or this August (at least in part).