Well I can report on it's progress so far, after about 7 years.
It has been treated with Brooks Proofide from new, probably 3 or 4 times per year, and has supported my backside for approximately 10k miles.
It looks ok for it's age, but has needed occasional adjusting to take up some sagging. Waterproof it certainly is not.
I doubt if dubbin has any magic ingredient to prolong it's life, but then I doubt if Proofide does either. Both products seem to make similar claims. The saddle is at a stage of it's life where it won't really matter if dubbin destroys it - but I doubt very much that it will.