What would they gain by upgrading you? You bought a Y seat and that's what you are getting and probably in Y+ since you are 2P. I never quite understand this question from a business point of view. They have those seats on the plane to make additional revenue for people who want to pay for it in money, miles or class purchased.

Op up's are another story but if you could choose a Y seat on a flight and know you had a good shot at F every time it would be less likely you would buy F. Then United wouldn't gain the additional revenue you could have brought.

I will also point out but not elaborate on airlines with unlimited complimentary upgrades. They are another story all together and are not usually available to HI.