I wouldn't be surprised, if the world lasts long enough, that they will become as large as the Roman Catholic Church is now -- or as large as some of the largest Protestant denominations.
Most people don't think deeply about religion, and they're likely to latch onto whatever comes along that gives them an emotional lift. I can visualize the rank and file in time becoming like church attenders and a special organization of "pioneers" forming into an elite leadership class that will be looked to as sort of a priestly branch of the organization. World headquarters will become a gigantic compound where Hollywood-style productions will be concocted to "prove" there is only one "true" religion and that JWs (perhaps with a new and more appealing name) are that "true" religion.