Alonso's break-out needs to be signicantly higher than any numbers he's put up in the minors so far to move Votto to LF and take that hit defensively.
It's not just about the bats-- gloves matter now.
Neither Votto nor Alonso are good enough in left field for a guy at first base to OPS 850. If you're going to take that defensive hit, you'll need a guy at 1B to OPS in the 900+ range.
So far, Alonso hasn't shown an ability to do that. His career minor league OPS is 837. He's got a marathon in front of him that he needs to run. He's just begun limbering up and some are insisting he's going to win the race.
Let's see him run first.