If either of Alexis or Villa is to exit, it should be Alexis because we already have Pedro to perform the role he does. Villa may not bring the stamina or workrate Pedro does, but he's an older player anyway and its not his thing to pressure defenders for 90 min. running around like a headless chicken.
He's a goalscorer. He needs to be in the box where he can get into positions. There is plenty of room for both him and Messi out there and there are plenty of goals to be scored. There is no reason that in the 2nd half of the season Villa should not have more of a protagonist role. He's not supposed to be the top scorer. But he is an excellent alternative when Messi is facing double marking, thus he can have some free space to do his thing.