Very short - AMD made an abstraction layer, to share effort between Linux and other platforms (i.e. Windows and etc.). Kernel/DRM maintainers don't like that, since it causes several issues detailed in that thread (harder to understand logic of the driver, slowdown of DRM improvement itself, indirect workflow of AMD developers and so on). For the reference, DRM here is Direct Rendering Manager[1], nothing to do with crooked Digital Restrictions Management.

1. https://en.wikipedia.org/wiki/Direct_Rendering_Manager