The concept of screen lockers is having a special layer, that can't be bypassed, which a locker creates. The whole security then hinges on the locker not crashing. X11 does have such a layer. Wayland compositors also implement it through such a layer. And for either the situation is, that if the locker crashes, that layer is destroyed by implication and the session exposed.

That's a flawed concept.

What you really want is detachable graphics session. On the text console one can effortlessly use screen or tmux and to "lock" the session simply detach and exit to the regular login getty.

You want exactly the same, but for X11. And there's no obstacle in printiple to implement this. It's just that the Xorg server can't detach. Almost all of the required code is there, fundamentally it'd be the same code that's executed during a VT switch.

In the meantime one can use Xpra with Xvfb to create detachable X11 sessions, which then however lack GPU acceleration.