I do not think there will be a relatively quick demise of JW numbers.
BUT I think that dropping the yearbook (especially as it could so easily be produced on line) is an obvious sign that the numbers are flat lining.
Dropping the average figure of publishers for 2017 and only retaining the peak figure can only have been done because the average figure looked worse. (maybe a decline) There is no other explanation that I can think of.
IF next year all we get is "highlights" then you will KNOW that even by JW "Peak" counting methods that they have gone into decline.
IF next year we get only the worldwide figure with a slight increase and no country by country measure you can be sure that the USA has gone into decline which - after this year's (2017) increase posted of +684 out of 1,232,293 = a "massive" 0.05% increase over 2016 - would not surprise me.