Rojas's platoon splits are a small sample, and also full of noise from his highs and lows. The Phillies and Thomson almost always ignore that (if they are platooning that is. They also ignore it when they have just decided a guy should play every day, whether they are right or wrong).
In CF as things currently stand it is pretty much guaranteed Rojas will play against LHP and Marsh against RHP. Even if Rojas continues to hit RHP better by his own standard, he's nowhere near Marsh's standard (or Kepler's).
And there's also no indication Castellanos will be platooning in any way. Nor should he, if he's able to avoid a slump and pick up where he left off last year. If he craters that's another story. And he should actually get one game a week off this year.
I doubt Marsh is subpar in CF (I.e. he will still be positive value). Just not as good as Rojas or as good as his own work in LF
Hopefully this plan works and they are not once again looking for a RHB OF or talking about Luis Robert. Crawford could still be traded, could be the 2026 starting CF or could just be the next Rojas. I'd be surprised if he's a factor this year (and if he is that means Kepler has gone the way of Merrifield).