Maybe it's also more subjective than we're applying to it? Like yes hope is very powerful overall and I would argue Gaiman used that as Morpheus's final blow as more of a general statement about its importance and qualities...but if we focus on who he's up against and who he is hope could make the most sense as the best counter attack.

Perhaps, if he were up against someone else or in another environment hope wouldn't be what he'd go for it got to an equally dire point in the game for him. You know what I'm saying? Although I do believe he believes in hope and, of course, his own hand in instilling that through dreams I think he also knew it would be what got under Lucifer's skin the most as well.