IMO the 'issue' is that Lacazette is being seen as a replacement for Sanchez, rather than as an incoming CF. As such he is the replacement & upgrade on the likes of Giroud, Welbeck & Walcott options NOT Sanchez.
Whilst AW played Sanchez at CF for the most part of the season, the fact that Giroud was used as soon as Walcott was injured & then Welbeck is IMO the real indicator that AW does NOT want Sanchez at CF in the current team/squad.
http://www.football.london/arsenal-fc/transfer-news/alexandre-lacazette-arsenal-arsene-wenger-13267872
Lacazette's conversion rate is supposedly not only better than anyone at AFC, but is also amongst the best in UEFA in direct comparison re expected goals etc = he is as good a finisher as the likes of Aguero & Co, yet he has a more flexible style of play.
The criticism re standard of defending in Ligue 1 is missing the pint in that it relates more to the ability to create goal scoring opportunities, and not about the ability to finish them as an expected goal is an expected goal. The only relevance then becomes the standard of GK, which again is essentially irrelevant re expected goal scoring opportunities (which are basically 1v1 on the GK, where the player is expected to score).
The confusion re Sanchez is IMO a distraction that can only be resolved if & when Sanchez departs, at which point Lacazette alone can not provide an upgrade or replacement for 2 roles. As it stands there is reason to think that Mbappe / Lemar are the more likely replacements for Sanchez, and whilst neither will currently seem capable of replacing/providing his contribution currently, there is every reason to expect that their current quality combined WITH Lacazette will provide an overall upgrade.
