He's started more games than he's been on the bench for so by and large you'd say he's been our first choice striker for essentially at least 3 of his 5 seasons at Arsenal. In which case, how much he cost and all that other stuff becomes irrelevant because he's our first choice striker and what we expect of a first choice striker is far greater than what Giroud has delivered. His 13-15 goals a season isn't good enough. His tendency to go missing for large chunks of the season isn't good enough.
Giroud would be greater appreciated as an Arsenal player if he'd spent the last 5 years solely as our second choice. But because so much of his stay here has been spent as a starting striker he's essentially following in the footsteps of some incredible strikers and he just falls so short of that.