Am sure @Vesper has drawn up a list of potential replacements for Willian back then somewhere. But you're kinda focusing only on the downside had we sold Willian when we could have easily gotten the big fee for him and gotten someone younger and better. And let's face it, Willian is consistently average. He will put in 5 exceptional performances out of 50 games per season and the rest he just doesn't do much but people will think he's great and all that.