I don't expect Phil to be better than Book. In fact I would expect him to be quite a bit worse. But the main thing is that he could be worse in a way that still helps the team.
We know what he isn't good at, but when defenses have to try and stop what he is good at, it will likely be in a way that helps our offense because they would likely have to keep players deeper than they are now, play soft coverage so they don't get beat over the top, and keep someone on Phil for the QB runs. All of that should make it easier to run the ball than it is now because we won't be facing a whole defense that is loading the box and pressuring guys at the line. It could also potentially open up some on those WR screen routes where we can better utilize the size of Claypool and the speed of Lenzy. And if teams would rather load the box and stop the run, all it takes is a couple deep balls for quick strike touchdowns and the game is already out of hand enough for it not to matter.
And I understand them saying Phil isn't ready because he can't read coverages. But there are ways to mitigate that, the simplest of which is probably just going fast with a check-with-me play calling. That system would likely also have the effect of putting us in advantageous mismatches because of the flexibility of guys like Kmet and Tremble. On top of that, we know Long calls a lot of plays that are designed to go to one guy, so there really shouldn't even be any reading of defenses on those play calls.