While the Phillies pitching braintrust certainly believes there is more there, I also don't think there actually has to be more there. And I don't think they expect him to start lasting longer. The bullpen they built is designed around the fact that Suarez, Walker, Painter and the other guys will not ever pitch the 7th, and only rarely the 6th. And the braintrust probably wants it that way too.
Walker's 2.6 WAR would have been third on the team last year (just ahead of Ranger) and well ahead of Eflin, Falter and Gibson combined (even if their low WAR was, respectively, a product of injury, part-time usage and being really bad for half the year). He's already a well above-average fourth starter, if not a third.
But Painter could still be better. A #3 or even a #2 if he has a ROTY type campaign in him. They are equally important in that having both of them, at the hoped-for level of performance, would give you a rotation with no 5th starter type at all.