Nothing is guaranteed to go up in value. It's a combo of supply and demand, which includes Marvel's decisions to reprint. If you had a Daredevil by Miller Omnibus, it was worth a lot. Then they reprinted it and it went back down to just being worth less than cover price. Then that reprint went OOP. Prices skyrocketed. Then another reprint was announced and now it's back down to below cover price. Meanwhile, Marvel has never reprinted stuff like the Silver Surfer Omnibus or the Wolverine Omnibus, so those stay sky high.
So if you have an in demand Omnibus or hardcover that goes out of print and Marvel doesn't reprint it, then theoretically it might be worth something someday. But if you have something like the Marvel Now Omnibus that no one wants it doesn't really matter if it's in print, out of print, 10,000 copies out there, or 5,000 copies out there - there's little to no market for anyone to buy it so it's not really worth much of anything.