At least so far, some private groups have access to Widevine decryptors.

Widevine L3 are galore. Ahem

The problem is the quality.

Those streams are only like 6-10mbps bitrate. A regular blu-ray is closer to 30, and UHD can be well over 100mbps.

Depends, high quality UHD bitstreams from quality streaming providers are around 25 MB/s. That's ~50 AVC-equivalent.

UHD BluRays indeed go higher, usually 70-ish. But the former are already very watchable, and plenty of groups can get them.