The biggest downside to X11 disappearing is that a lot of remote desktop apps and headless sessions use it. VNC servers, X11 forwarding over SSH, and XRDP all rely on it, and there's no perfect equivalent for them in Wayland. But even that space is slowly shifting over to Wayland, with tools like GNOME Remote Desktop gaining traction and RDP support growing. It's not as straightforward as X11 was, but it's getting there. Most users on KDE Plasma probably aren't going to be doing serious headless remote session work through it anyway, and those who are should be able to find viable alternatives