Jan: Like the GB doesn't really approve.
That is exactly what they want the rank and file to feel. It's a sort of "you can do that if you feel you need to, but Jehovah would be very proud of you if you didn't" attitude.
If they did say that Christmas celebrations were a conscience matter, a lot of JWs would still not celebrate it. My mother would not, I am certain of that. She would point to all of the old WTS teachings and lean on those. But overall, I think there would be a gradual shift towards celebrating. The issue for the GB is that it would likely cause rifts in many congregations, and lots of brothers and sisters would feel like they had been betrayed. If there isn't that much to differentiate the WTS from other Christian denominations, why not go to one of the others?