Have MG not stated new builds will have the breather. So it's a question about the built cars being flushed out of the system.
Have they got the volume of production of the breather to fit.it on all cars, and as
@siteguru says do they need the dealer network to do the fix.
I've nearly 700 miles on the clock no leak yet. My dealer was going to delay my delivery as it was around the time there was debate about all cars getting the breather. I needed it as I had sold my existing car and so persuaded them to let me have it. A delay would have caused me frustration and I would say would be poor customer service, the point I'm making is I think there are many things being weighed up and no answer that will make everyone happy.
I'm impressed in many way that they are resolving problems relatively quickly (even if it doesn't feel.like.it sometimes).