On the surface this makes a lot sense for Arsenal.
A free transfer for a cash strapped club. Check.
A creative wide player. Check.
A good ball progressor. Check.
Connected to Kia Joorabchian. Check.
There is one really big problem and one that should override everything else, his age. Willian is 31 years old and would start the 2020-21 season at 32.
Looking at Arsenal’s age breakdown that puts him well among the past the prime players that Arsenal have on the team, many of them who Arsenal are actively trying to move on from.
Looking at what Arsenal have in the wide attacking midfield position, Arsenal have Pierre-Emerick Aubameyang just exiting his peak years (but still easily the best attacker on Arsenal, but also with his own contract question hanging over the club), Nicolas Pepe just entering his peak years, and then a bunch of players coming up from the academy.
With this it does make sense that a more established player fills a need.
Given Arsenal’s current position in mid-table, with a minimal transfer budget (probably for the near term future) signing a player on the downturn of his peak to an expensive contract with a minimal transfer value has disaster written all over it.
If Arsenal were on the cusp of fighting for the Champions League (at best next year they will start as the 6th best team next year but probably lower) or the League title (lol) a signing like Willian, where the focus is to put the team over the top for a huge payday or glory for the next season is worth the risk of the back end of a contract looking really ugly.
It almost doesn’t matter how good Willian is (he isn’t at a World Class Level™ where there are different rules) because of where Arsenal are in their current talent cycle. More short term thinking will just extend the current malaise that Arsenal find themselves in.