Messi can and does play passes to the right too. All you'd be doing is switching his passes to Alves to the other side with Bale. It wont give him more than he has.
Having watched every single Barcelona game in which Messi has featured, I can assure you that like 95 % of his passes go to the left side, often inbetween rightback and right centrehalf. As a left-footer this comes more natural to him. He even assisted Abidal. Twice. Now Abidal has 2 career goals for Barcelona. Can't remember the last time he assisted Alves though.
Bale should also be more prolific than Alves. Alves scored 3 goals this season in all competitions, despite often playing as a right winger. Especially this season when Pep used a 3 man backline pretty often. 1 of them was a wondergoal.
The downside is: Alves is actually a good defender. A very good one. Bale isn't.