I really think there is no reason to reinvent the wheel here, it's just a matter of keeping Turnbull on his preferred routine and getting high-leverage work out of him versus burying him as a Covey-esque long man. Of course if he is better than Walker and Sanchez he should just start, with Sanchez as the likely loser there due to having more relief experience and recent struggles. But they still need to see how Walker, who was an above-average (leaguewide) pitcher last year, does.
I don't think there are egos and contracts involved with Casty but there definitely are with the rotation. Walker is a high-paid vet who needs/wants to start, Turnbull is a career starter whose contract rewards him getting starts (though it's so little money the Phillies should just tear it up and pay him $4 million regardless of his role).
I think the May concern is actually the bullpen. The offense is a known quantity whether certain players come around or don't (and others possibly slump or don't). But the combination of carrying Turnbull, Seranthony's struggles and Soto, Marte and Kerkering not being consistent leaves it a little thin if certain people are forced into bigger roles or workloads.
But at the same time May is still a bit of a shake-out cruise in that regard. Still a few fungibles in Allentown who might get a look, as well as the possible return of Ortiz (who can be optioned) and Covey (who can't and is presumably gone but you never know).