UA could, of course, start service at the rear, increase the number of carts, and any number of things. But, the chances that it will stiff theoretically more "valuable" fliers, e.g. those who are in E+ through status or paid seat assignment, is zero. If someone is going to end up with the short end of the stick, it won't be E+.
As to increasing stock, while UA does not have an absolute metric, it of course tests increased supplies until it hits a point where it can't sell. While chips and the like don't wind up in the trash bin, anything fresh (sort of) goes in the bin at the end of the flight for food safety reasons.
Safest for OP's situation is to bring with whatever it is that you want. Given the quality & variety available at many airports, even if flying domestic F, it's often better to buy what you want.