I think that's Phil's underlying thought. Without him, who keeps the team together? Daisy is the only one who could keep them from all drifting off into their own little dramas. She's the only other truly unifying force in the group. We've already seen FitzSimmons, and May, and Mack tempted to drift away from SHIELD. But could they leave Daisy? She's become, along with Coulson, the beating heart of the group.
So it doesn't have anything to do with being ready. She can learn those things from the people around her, and have her worst impulses tempered by their wisdom and experience. It has more to do with being the only one that can keep the rest of the team together and unified.
Well, I think it was mentioned in an earlier episode that what disqualifies May from taking over is her leg injury; she's no longer able to lead a team into action. (Unless she wants to trade up for one of those leftover android legs they have lying around now...) But she's taught Daisy everything she knows.