I've said it before, but when UA puts a dynamic / variable price tag on a seat, it should be fair game to select it and remain in it -- regardless of any other factors. There's a reason E+ middle seats are cheaper than Window and Aisles -- United knows they are less valuable. So if United (or any airline) is willing to take my additional money to choose a window or an aisle, they should have no issue with me getting what I paid for. Now if we are talking a free seat selection in E- then absolutely they should control it how they see fit. But once someone is paying a dollar amount for a specific seat, especially when all dollar amounts are different, you should get what you booked.
And just a case in point from my personal experience -- when I book aisle aisle or aisle window in E+, of course I'm hoping for some extra space. If someone (let's call it a standby) gets my middle seat, I always offer the courtesy for them to have the aisle or window. BUT -- my grandparents for example (mid 80s) book aisle window and they 100% of the time sit apart from each other even with someone between them. They don't want to talk. One wants easy bathroom access. One wants to look outside. Simply put, you should get what you pay for.