It makes perfect sense for Arsenal to sell, but whether they will or not is still up for debate in this crazy summer.
The smart money will be on this dragging on until the final day, with Sanchez being 'injured' or not deemed 'fit enough' to play after the Confeds Cup. We'll settle on a price & Arsenal will give Monaco the £50m they want for Lamar & Arsenal can blame the whole thing on being held hostage by Sanchez & having no other option but to sell.
However, would you bet next months wage on this definitely being the case? I wouldn't as Arsenal don;t exactly need the money & could afford Lemar regardless.
For political reasons Arsenal could make Sanchez see out the final year of his contract in the hope that a front 3 of Lemar, Lacazette & Sanchez will get them back into the CL next season & probably help pick up a cup or 2 next season. It's a distinct possibility mate which is why I never get twisted over transfers in the window... Isco finished me off! Loool :-)