christianity decline from a christian perspective

[Mister Agenda]

Because this is the end, the organisation(s) has/have lost connection with the younger generation and thus have entered a terminal decline which will follow an exponential path in line with average age of the congregation reaching life expectancy.
Of course the organisations themselves are still in the denial stages.
This will result in no action being taken till almost the point of no return and when it is it will be opposed by the remaining members ensuring that the organisations are consigned to oblivion.

I doubt oblivion, but long term or permanent minority status, as is happening or has happened in much of Western Europe. Otherwise, I agree with your analysis completely.