It already is a fringe religion. Just a minor Cult.
I can see their membership shrinking more over the next few years, the "increase" they are getting comes from the few places in the World where the Internet is not available to people, or as has been said, from their offspring.
The young ones in their late teens and early twenties in the West at least, are already not really committed, they view it as a huge Social Club. Sadly they often end up married within the Org, and perpetuate the cycle. But the real die hard 100% believer J.W is as rare as Rocking Horse poo today, meaning that donations are not likely to be anywhere near as high as decades ago.
The Business Model the org has is pretty lucrative, buy a plot somewhat out of town, when it goes up in value because the area has prospered, sell it. Rinse and Repeat. This is getting harder though as they do need to seat their members in a K.H somewhere, and excess travelling cuts attendance.
They are probably happy for it to shrink to about a 5 mill. membership, but who knows what effect the Abuse scandal etc may have on membership ? and as the Internet spreads retention will be much harder.
Maybe another big scandal will damage them even more in the future, a financial one for example.
I see them as hanging on for a few more decades, but becoming as irrelevant and almost unknown as the Plymouth Brethren etc.