You're right, problem is we don't have the player for that situation (outside of Messi obviously). Neymar can moonlight there, but Neymar is not an everyday CF type, he's not physical enough nor does he have the movement required of a CF. He'll get beat up and his production will dip the longer he stays there game after game.
Alexis is not a CF type for this team. He can play centrally for Chile because he has an ungodly amount of space and freedom there. For Barca he does not have space at all, and his first touch is poor and his lack of balance would have him falling a lot from contact of the ball in tight windows as well as the CBs.
Playing Pedro as a CF is a monumental waste of your best scoring winger. He has movement and first touch, but he's just far, far, far more efficient making runs and creating mismatches in the wing.
As less than ideal as Cesc playing false 9 is, sad to say it, he is the most efficient option out of a whole bunch of inefficient ones, because he can at least give stellar performance out of the midfielder part of the role, and he is a very underrated finisher. This is why the team needs another CF aside from Messi.