The whole concept of the 'main striker' in this context is absolute rubbish. It just another one of the these simpleminded football tropes that makes it easy for the common fan to generalize.
Just because Sanchez was coming off the left doesn't diminish his goal scoring responsibility. Alexis averaged 3.6 shots per match throughout the season, that's in the same bracket as Aguero, Kane, Vardy and Lukaku aka best forwards in the league and it's actually MORE than Giroud's average shots per match throughout the season yet Giroud scored more league goals than him despite playing less minutes. What does that tell you about how was more efficient in front of goal?
All the forwards around him league goal returns are in the high teens whereas alexis is on 13 which was largely padded by a goal flurry against bottom half opposition after our league chances were already dead and buried. It's clear Sanchez failed in front of goal and he's the one who missed too many chances.
If anything Giroud is the only Arsenal attacker who barely did his job properly. Sanchez and Theo both went missing for large part of 15/16, their goal return was incredibly poor.
Once again with Ozil you make excuses as if Giroud was the only offensive player in this team. He has a has front 3 to utilize