The issue is that 16 is nowhere near enough for the starting striker of a team with title ambitions. One thing Wenger has always made sure we have had is a top class striker, until now.
Who was top scorer at Chelsea last year?
Only 5 players scored more goals in the League than Giroud last year. 4 of those come from 2 teams, and the other one was a designated penalty taker for his club. Goals scored doesn't tell the whole story nor does any stat taken in solitary context. Giroud makes us a more dangerous team going forward, as even if he isn't scoring he is making space by dragging defenders for others to score.