No, it's not impossible.
The lockscreen is part of a systemui plugin.
Removing this plugin makes the lock switch not function anymore, and the screen no longer gets disabled when it's dimmed/blanked (touching it turns it on).

To replace the lock screen, one would have to replace this (vital) part of the system first, then other GUIs can hook onto it, the rewritten plugin should send the same DBus messages that the current plugin does so it doesn't break apps.