Funny, the way I see Castellanos's season is that Thomson believed letting him battle through it and adjust was the best thing for his game, and that approach was entirely vindicated when he was rewarded with an elite offensive OF from June 1 on (129 WRC+). Nick was the Phillies best offensive OF and third-best hitter (even over Turner and easily over Bohm) from that point. He was also quite consistent from month to month after April and May.
Whereas Marsh, whom I quite like, had just a .703 OPS in the second half (.781 in the first) and would actually lose value/have a lower WAR if he played more/faced more LHP. IMO. He was unplayable against them last year (.552 OPS). It's a tough thing because he'll never get better if he doesn't face them more, but then you are taking away ABs from Rojas or Wilson, not Nick, who should always face lefties.
I don't know if Nick can avoid another slump or be that good again one year older, but I don't know that about Bohm either (including whether he'll really grade out as well defensively again). I don't necessarily agree with the Phillies preference to move Marsh out of LF (which is a done deal now it seems) or limit Rojas's play as extremely as they have but this offense still needs Castellanos.
What's ironic is offense from the OF is down all over the league, which reflects really well on everyone. Marsh and Nick are both more productive at the plate than their detractors think, and Rojas's lack of offense isn't as big a deal given his defense.