The proprietary NVidia driver now supports gbm, which vastly improves the user experience. For older devices that will not get gbm support dropping EglStreams will likely not have a big impact as it has several session breaking issues anyways.
By removing the backend a lot of logic can be simplified, most notably multi-gpu. In addition with introducing OutputLayer and making subclasses of it responsible for a bunch of the render backend stuff I think the simplifications will be very significant.
The only question is: should this wait for 5.24? In practice the wayland session is borked for most of our users right now anyways with the egl-wayland + Qt issue and the world hasn't ended, so I think it's safe to say that the amount of actual users of the EglStreams backend is very low.