I suppose in theory they could stay reared up browsing for a while, but every mouthful that needed to be chewed and swallowed in that position would be expending at least some muscular energy in stabilizing itself and maintaining leg extension.

If it was swinging it's neck from side to side while upright, then it would also need to be expending energy not only to do that but also to shift it's weight to counterbalance.

My intuition says it wouldn't be worth it, and the size some of these dinosaurs grew to suggests that easy (e.g. ground level) food was plentiful back then. I've always supposed that evolution bred gigantism out of most DNA lineages as a hard won lesson that food won't always be plentiful, and that in times of shortage being smaller is an advantage.