Also you aren't going to be playing him up front.
Giroud was important last season for his hold up play and his flicks, you're going to lose a lot of height and physicality in the team if you play Sanchez up front, meaning that you'll have to rely solely on controlling possession of the ball.
Don't think Sanchez is the right man for Arsenal, but is an improvement on what you have. I see an Ozil situation happening. Him going ham for a month or two and being really poor until the end of the season.