People on Unix environments that don't have Wayland support.
That's a big one. All the *BSD folk will keep on using X at least until it gets proper support over there (which might never happen) and even then it will still be boycotted by some BSD users for other reasons.
People using mainstream desktop environments that already support Wayland ... [but their distro hasn't made the switch]
I agree about that. Many people don't care and will just use whatever their distro tells them to use. As you said, there's usually good reason for it.
People using desktop environments or custom X setups that don't (currently) support Wayland.
This is another one, and is actually one I kinda fall under. I use a tiling window manager. The tiling Wayland compositors are often times not as polished, and a big annoyance for me personally is the fact that most of them (River, Hyprland, DWL) don't come with a bar. Of my X Window Managers, AwesomeWM, DWM and Qtile already have their own bars. BSPWM is basically supposed to be used with Polybar, the same way XMonad and xmobar are basically made for each other. On Wayland, Somebar is made for DWL, but waybar and yambar work really well with it. Sway has swaybar, but waybar works perfectly with it. Both Waybar and yambar work great with River. And there's Waybar, and gBar, and other bars for Hyprland. And that's without mentioning EWW, which can be used to make a bar.
Another issue I have is that my touchpad doesn't get detected if I'm holding down a key. So if I'm playing Minecraft and I'm trying to turn around and run away from a zombie using my touchpad because my mouse's battery ran out, I have to do these actions one by one and hope I survive, or just let myself die. That's just an example, but I have noticed it in other games as well. No such issues on X. And I've also had Powerwash Simulator, ran through wine, just crash on me in some (Qtile or Hyprland), but not other compositors. In DWL, I couldn't turn all the way around and forbsome reason my movement was restricted to 270°, and in River I had 0 issues.
When you have a monopoly
You're saying this as if X didn't have a monopoly over Unix graphics.