The number of people self-identifying as JWs, which is what is reflected in census data, exceeds the number of people who are click-the-box publishers. There are loads of people out there who come only to the Memorial, but are former publishers/ family no longer actively associating who probably consider themselves JWs and self-identify as such.
I think an argument can be made that the life cycle of the JW religion in a given country goes through a demonstrable process occurring over several generations.
1) First generation of believers commit to the lifestyle and are relatively successful in raising their children in the same faith. (high growth rates, 1950's through '80s in the US) (I realize the religion had a long history before the 1950s, but it went through a re-invention post-Rutherford, in my opinion)
2) Second generation commit at first,but by the time they are in early middle age and having to raise their own children, stay in nominally but realize they are screwed and are glad to see their kids decide on their own to leave.(still showing growth, 90's through 2010's)
3) Third generation is just hanging around and are non-committal, though they still may self-identify and show up to memorial to appease grandparents and parents. (2010's until present, stagnation)
The US is in stage 3. Some of the African nations like Congo and Nigeria are still in stage 1, hence the vibrant growth. Japan is entering stage 3, Mexico is in stage 2 heading into 3.
Where stage 4 will lead us in coming years I don't know.