I've actually been wondering if some sort of unification will happen.
The M1 graphics stack seems like it borrows a lot of code from the iPad instead of the traditional Mac graphics stack. That means it's more modern, but it's missing some of the usual niceties of the Mac stack. It may be linked to why M1 Macs seem to have resolution issues with external displays. Could also prevent devices like eGPUs.
On the Mac, the stack was pretty ancient with some parts still dating to the NeXTStep era. But it dealt with a lot of corner cases and display management that the iOS stack likely didn't. The stack is a lot more than just GPU and graphics output. It also handles the management of displays like arrangement, scaling, and resolution.
That could end up being unification with the iPadOS (bringing better external monitor resolution features it iPadOS) rather than letting 3rd party GPUs back in. If most future iPad's have USB-C sockets then hooking up monitors ( or docking station monitors ) could become more common for iPadOS.
But yeah. Where Apple was going to go long term with GPU drivers was bound to be one of the last things they did has they moved to kill off the legacy kext frameworks. Even if 3rd GPU drivers came back it it was likely going to be radical rewrites and far more Apple controlled. Apple would probably need at least a year of "try out these beta alternative" drivers, but they'd be ready for prime time. (and a decent time a some NDA bound alpha stage where Apple thought they worked well on a too small monitor sample set. ) . Two years to get to a beta wouldn't be surprising. But after WWDC 2022 , if they haven't shown something then they are probably more interested in iPad unification.