Correct me if I'm wrong but to stream 4K, studios require a hardware root of trust and a verified media path. They need a guarantee that the video frames are decrypted inside a trusted execution environment and sent directly to the display without the OS kernel or user space being able to read the raw buffer.
AFAIK Windows and macOS provide this pipeline at the OS level. OTOH, ChromeOS gets 1080p/4K not because it has massive market share but cause the hardware and boot chain are locked down by the almighty Google.
On desktop Linux, where you have root access and can modify the kernel or compositor to inspect memory, there is technically no way to guarantee that secure path to the studios' satisfaction. Am I right in this assumption?
Unless the DRM providers change their threat model, which sounds unlikely to me. Or distros start shipping signed and locked-down kernel modules that prevent the user from being root, which is again unacceptable to most (me included), we will likely be capped at 720p for some time now.