In the UK, regular churchgoers account for about 12% of the population. Some of the other 88% may tick 'Church of England' on the census form out of habit but I think it's reasonable to conclude that a significant majority of the population of the UK doesn't believe that Jesus was the son of God.
As for "Jesus coming in the clouds to save everyone who believes- and kill everyone who doesn’t", I would say less than 1% believes that - this isn't a doctrine that the Catholics or the Church of England actively preaches.
I'm hearing people talk very negatively about religion as a result of the rise of Islamic State. It's not just Islam that is coming in for criticism but all religions with their received wisdom and absolute certainties. People seem generally far more sceptical and prepared to challenge these days and while no doubt there will be some believers for many, many years yet, at least in the UK I would expect Christians to become an ever smaller group.
JWs may not suffer as badly because of their shunning policy holding people in. Even then, it's one thing subscribing to a particular religion when most other people are religious, but quite another when almost everyone else is secular - the feeling of otherness must grow for the still-in Witnesses.