All software/hardware in the system (Core Animation, Retina scaling, HDR, Apple Silicon's GPUs) assumes an isolated/composited display model. A model where a modern rendering pipeline is optional would add major complexity, and would also prevent Apple from freely evolving its hardware, system software, and developer APIs.
Additionally, a mandatory compositor blocks several classes of potential security threats, such as malware that could suppress or imitate system UI or silently spy on the user and their applications (at least without user permission).
All software/hardware in the system (Core Animation, Retina scaling, HDR, Apple Silicon's GPUs) assumes an isolated/composited display model. A model where a modern rendering pipeline is optional would add major complexity, and would also prevent Apple from freely evolving its hardware, system software, and developer APIs.
Additionally, a mandatory compositor blocks several classes of potential security threats, such as malware that could suppress or imitate system UI or silently spy on the user and their applications (at least without user permission).