I considered it but I thought that "to descend" was too rare and formal in English to really carry over how the phrase truly feels in French. "I descend down" decidedly feels much more awkward than "je descend en bas" to me, if only because I've heard the latter a lot and the former probably never.

"I climb up" might give a somewhat better impression of how "je monte en haut" sounds in French, although I suppose that you can very much "climb down" in English so it's not a perfect match.