Hate to be overly practical, but what about cost and quality?
Aren't good-quality G horns harder to find, not to mention repair, than Bb/F horns?
Aren't they more expensive to buy and repair?
I knew someone in the repair business who remembers trying to find parts for G horns and having trouble.
Besides, at the risk of sounding sacrilegious, didn't hornlines sound better after making the switch to Bb/F?