That’s fair enough, Pew provides some evidence for that conclusion. It needs to be qualified because the number of JWs in the sample was very small and has a large margin of error. JWs rely on converts for growth to a greater extent than most churches, so this stands to reason.
Groups that have a stable community identity, such as the Catholic Church in Catholic countries, and Mormons in Utah, have high retention because members are surrounded by other members and social pressure is to remain a member or to identify as a member even if inactive. For JWs, outside of a few communities, perhaps in places like Guadeloupe and Zambia, JWs are always a minority in larger society, so there isn’t that same pressure on that front.
Mormons outside of the United States clearly have very low retention rates. It is common practice for missionaries to visit people for only six weeks before baptism. After baptism some new members never see the inside of Mormon church again. This is turnover on steroids and is the reason why the Mormon church ends up claiming 500,000 members in Chile while only 100,000 describe themselves as Mormons, even after missionaries have knocked on their door to try to remind them they were baptised as Mormons years in the past.