Pretty sure Giroud has one of the highest goal scoring rates this season!
Also you said Costa has scored many match winners/important goals but so has Giroud. So many of his goals were far from just "tap ins" and not all of them were just being in the right place at the right time. Guy has created his own chances and scored some great goals too. Costa has an advantage with his pace so is more likely to score more goals but Giroud's integration into our team is vital.
To say he's not good enough to lead our line is a false statement however. Striker is not the one position of attack. If we want to score more goals we need better wingers too (only natural wingers we have in the club are Sanchez and Walcott, one is a one sided player and the other is inefficient). I would advise buying a better striker but not to replace Giroud, instead to play with him up front. Have sort of like a two striker system but Giroud drops back anyway and is important to our passing so can't really go wrong!
It's a shame he's hit his peak at such a late age though. Hopefully his decline doesn't come too soon as I'd like to see more of this Giroud.
As for you thinking he's not good enough, how many of Giroud's goals have you seen this season?