I think they will send some animals to other zoos in two or latest in three years. As you said they are breeding well and the capacity of their pen is limited, even if they expanded it last year through connection of the muntjac´s pen.
They already sent a bull to Chomutov a few years ago.
The problem: the founding base is extremely narrow; inbreeding is almost unavoidable, unless (which is rather unlikely) they get new specimens from China.
@Trebaruna: Point taken-yet you could say the same about pretty much all larger species kept in zoos, even (or rather, especially) the ones "important for conservation".
However, it shouldn't be the way to go if it can be avoided...
BTW: Bhutan takin (Budorcas taxicolor whitei) kept in any Western zoos at all? Motithang Zoo seems to keep some...