Who knows what the future holds, but if decline was going to set in you might have reasonably expected it to have started already: the generation prediction failed in 1995, the Internet has been exposing them since the late 1990s, bad publicity on child protection in the mainstream media, book study cancelled in dubious circumstances, numbers of anoined on the increase, and goodness knows what else. Yet they still posted an all time peak for publishers in the UK this year. If all that didn't result in a decline then what exactly would it take?
I firmly believe that if they were measuring today exactly what they were measuring 12 years ago, you would see a decline. Interventions such as allowing people to count as pubishers by recording 15 minutes of time and allowing both parents of chidren to count as active pubishers by both recording time spent studying with their children have fudged the numbers.
I can't speak as to what is happening in other areas of the UK but in our congregation, we have had only one convert under the age of 60 in the past ten years that I can recall. Silver surfers are the fastest growing demographic on the internet though and I think this last stronghold of potential converts will become less productive in the coming years. Incidentally there is not much in the way of a foreign language field where I am and the picture is probably different again in those areas.