Nope - that isn't the reason as other studies using a less leading question and therefore without any need for a campaign for honest response have found similar declines (albeit typically with a greater proportion of non religious) e.g. BSAS.
Here is a hint:
1. Are the oldest generation more religious than the youngest generation.
2. Are the oldest generation more likely to die than the youngest generation
3. Do people typically become more or less religious as they get older.
I suspect questions 1 and 2 will be obvious to you Vlad. I suspect you will make a rookie error on question 3 as you misinterpret the explanation for your answers to questions 1 and 2.
I think I've said I prefer your prediction that only 6% will respond as christian in a few census time rather than JeremyP's even more dismal but unlikely 0%.
However the census asks about religious affiliation so the 6% is going to be bolstered by nominals and believers but non attenders ( people with Kids, People with Jobs, people just too shagged out at the end of the week ) which segues us into your question of the old and the young.
Anecdotely i've always known elderly people returning to church, widows and the like since there is guaranteed company and i've known people return with age because they have thought about mortality and what not. Take away social services and elderly care and I think you may find that people will drift back to churches. Remember, you are talking about sociological surveys and giving them the predictive power of pure science
As for the young, I think they are a constant surprise so I don't think we can ascribe social surveys the predictive power you are giving them.
Finally, triumphalism on the part of anti religionists is a thing. I would move you have it, JeremyP has done it with his predicted no one will be a christian and Simon Jenkins of the Garden who, at 46% of census responders identify as Christian has suggested that the church hand all churches over to local councils. That is ideology smuggled in under the guise of spun statistic.
But then Jenkins knows that churches are often refuges in times when the state lets people down and wants to nationalise and secularise hope although we know that the councils would shut the churches down like libraries and other utilities and sell them off for houses
I agree that there is a decline in christianity both in this country and else where However it is a very British decline, alongside that of health provision, politics, pubs, giving a shit about the number of people who died of Covid. I understand though that ''The young voted for Matt Hancock'' on I'm a celebrity get me out of here.