Coming as an Arsenal fan, I assure you Sanchez will not be joining United. The revolt will be catastrophic and most Arsenal fans would expect Arsene to retire immediately. Simply put, a transfer to United regarding Sanchez will have a bigger negative impact than City.
Based on my assumptions, I also think Sanchez wants to swiftly return to the very top (Like the Barca days) doesn't want to wait a year or so for the United project to finish. Guy seems to be impatient and itching for a move to what many would perceive as the best team in England.
I would be very surprised if United have even a sniff of a chance. With our other players? Yes. With Sanchez/Ozil? No.