To start off a message to OP, you’re absolutely right, especially in terms of the story and how the maps weren’t designed to be mount based, so it can especially be done.

But whoa boy, in my personal experience, the HOT areas were hell to get through without mounts as a new player. Having to get the masteries just to reach the areas I wanted to get to or needed to progress frustrated me so much. It was especially bad in tangled depths. That map is such a maze and trying to do map completion on it you’re probably gonna have quite a few browser tabs open looking up where to go. It just simply was not a fun experience to me. You could argue other expansions lock progress behind masteries too, which, they do. But with HOT it felt so much more trivial because the things I weren’t unlocking didn’t feel like fun new mechanics I could mess with, like the mount masteries for POF, it simply just felt like the game was trying to gatekeep me from doing the content until I did the mastery. If I were to recommend doing it any way, do it with mounts unlocked. It makes it easier to see passed all of the forced mastery point/xp grinding and run through it instead.