50GB is a lot of downloading for users with usage caps and overage charges or services that can't download that much in a non-frustrating time with zero errors.
Since there has to be physical distribution for at least some users, I agree with you: BD is the best available technology. The cost of a new or additional Blu-Ray reader should not be an obstacle for anyone with the iron it's going to take to run this game anyway.
Gonna disagree with you here, I'd prefer a bunch of DVD's to Blu-ray, unless it starts to be common for a LOT of games to be huge. My download speed from Steam is around 750k so if the game is big, I'd definitely go for physical media, but blu-ray players aren't exactly common here, and it would be an expensive purchase for just one game.