Football transfers are generally quite crazy and are affected by all sorts of factors. The bottom line for me is, I'd rather have Pogba than have the £89m sat in the bank not doing much, or waiting for the Glazers to use it for something else.
I think the marketing side of it will likely be greater than you've suggested there, but I'm not saying it'll necessarily be huge either.
As it stands, I have no issues at all, but I do agree it's a lot of money. To put it in perspective (flawed though this comparison may be): would I rather have Pogba, or Sterling + Andy Carroll - adjusted for market inflation... easy decision.
Again though, if he gets injured or struggles, it'll suddenly be a disaster, but that's a risk with any big signing.