[arghwhat]

"just had a framebuffer" != Using fbdev. fbdev is a specific outdated API, which drivers only emulate for compatibility with e.g. fbcon (the kernel's terminal emulator).

KMS is how you control the display hardware formally on FreeBSD and Linux. Other platforms will have their own APIs for doing this, but you are writing all the bits of a display server for that platform.

And, well, a small kiosk Wayland server is basically just KMS + libinput + a tiny IPC layer. All you save by doing this yourself is the last bit, but at the cost of having to deal with e.g. display modesetting and GPU buffer management.