"chrustaedlohians"
That's gotta be the worst case of "please excuse any errors, I'm typing this on my phone" I've ever seen 😱
I'm assuming "Christadelphians"? 🤔
They actually experienced (or seemingly experienced) a resurgence in the nineties. The newly emerged internet was estimating their number as about 100,000 worldwide, whereas the "anti-cult" literature of previous decades only had them around 20,000. But like most Christian denominations and -isms, they've since declined, to about half that (50,000), according to Wikipedia. Australia holds the dubious distinction of housing the highest number within the Anglosphere. Not surprised, actually, I've run into a few over the years.
The decline of church attendance is real across the board, but as TonusOH above says, in this case, the obsolescence is built in. They used to be able to "reproduce" by adoption, but governments frown on adopting kids into an overtly religious environment these days, so that source of "born-ins" has dried up.
Does the article* mention whether the two survivors were adopted, or were they converts?
*It's behind a pay wall.