I streamed it on a TCL Roku 4k HDR set with the native HBO Go app. I had no issues with visibility or clarity and my set is calibrated correctly at eye level.
I absolutely believe that most people watch TV on a washed out, out of the box Best Buy configured flat screen that they've mounted over their fireplace that's positioned too high. People think that watching something at a 30 degree angle in a brightly lit room is optimal, it's not. Fuck those guys.
I'll agree that it was pretty dark, but this is a completely subjective argument. If the cinematographer, color corrector, and editor say it was to their specifications, then they wanted it that way. Plain and simple. If you had issues with the viewing, it's going to come down to QAM/stream compression, lack of calibration, ambient room luminosity, and viewing angle. Or maybe you're fucking old and getting blind.
Edit: Only true test will be with BD source, but we have to wait for that.