For reference, the IEEE 802.15.4 spec is ~800 pages long. 900 pages does sound like a lot considering that Matter (AFAIK?) doesn't directly spec any hardware or transport details - those being covered in 802.15.4 and Thread.
Granted, we should remember that those 900 pages include base details that, probably, CSA are not planning to change in the foreseeable future. They need to be very thorough.
To answer your real question: device manufacturers will likely use the Matter SDK. It would be a huge undertaking for a smart-light manufacturer to re-write all of that code from scratch!
I haven’t read the spec, but I believe that some backwards compatibility is built in, and there’s a degree of complexity involved in making a robust framework that isn’t general purpose (like Wifi), but instead offers compartmentalisation of different device types and use cases.
I agree with the manufacturer concerns, but many were in a bit of limbo while Matter and Thread languished in draft RFC hell. The spec was always ‘coming by the end of the year’, and obviously impacted by the pandemic.
At least now, there is some certainty and a path forward.
Or phrased another way. Will no name, cheap devices ever properly implement this spec?