I am sorry to tell you Phizzy, but I know many churches that have declined far more than JWs and have never reached a point of saying “can this be the right religion”, or “is it worth it”, and “time to throw in the towel”. Well on an individual level I am sure some members do think that - and that’s part of the reason for the decline! But it never seems to reach a tipping point where there is a collective reckoning as such.
Churches that have experienced severe decline in recent years where I have some direct knowledge include Methodists, Church of Scotland, Mormons, Christadelphians, Unitarians, Baptists, Brethren, Swedenborgians, Christian Scientists, New Apostolic Church. I’m probably forgetting some. Some of these churches have lost half their members, or more, over the past 20 years. Yet until there is literally no one left to physically turn the light on and off (this happened with the Swedenborgians who closed in Scotland with 4 members remianing, down from 800 in the 1950s) there simply doesn’t seem to be a level of decline where a religious group will throw in the towel early. They just struggle on and readjust to the shrinking numbers as best they can until there is no one left.