Well, I'm not really stating my opinion, just what I think the Phillies believe - and what they already have been doing. I have no issue with either player, but DD has publicly stated they are likely to make a change in the outfield and he already set that path (playing Rojas less and Marsh in CF more) in the second half of last season too.
Personally I would have just played Marsh and Rojas every day and lived with both their batting issues. I certainly wouldn't have played Hays against RHP. Marsh is never going to get better against LHP without facing them, and it's the lesser number of ABs. Rojas also never gets better without playing regularly.
I agree with you about Casty too but changing his role or playing time seems like the third most likely thing to happen, and his bat is still pretty darn strong when you take away his slow start. Can he avoid that or even repeat his consistent .800 OPS performance from last June on? Maybe, maybe not.
Perfect world, they find a way to trade Casty and just buy a new OF. Even if they can't offload much of Nick's salary they're signaling they are going to bust past the third threshold any way, in which case it doesn't matter. But if he's still on the team, merely giving him a day or two off every week isn't enough to actually change the outfield mix.