I had not heard of the Libby (Montana) community before.

But from the description in the article, it is clear they are at the liberalising end of the Amish.

And one thing that almost certainly follows from their liberalisation, is their TFR (Total Fertility Rate) is going to gradually converge with mainstream society – not necessarily with the very low levels associated with the completely secular, but at least with the levels associated with mainstream conservative evangelicalism – modestly above the secular average, a lot lower than the Old Order Amish average.

By contrast, groups at the most conservative end of the Amish–e.g. the Swartzentruber–have a very high TFR, and it seems unlikely it is moderating to any significant degree; and also I'm sure their Pennsylvania Dutch is much healthier as a language.

Comparing Pennsylvania Dutch to Yiddish, I think the fact that Yiddish-speaking Hasidic communities (e.g. Kiryas Joel) use it as a written language, e.g. for their newspapers and community notices, and also a language of instruction in schools, puts Yiddish on a much more secure footing. I wonder why the Amish have never made much effort to write their distinctive language down? As far as I know, there isn't any theological objection, just a cultural habit they've stuck with. (They could keep standard German for their liturgy, just as the Hasidim use Hebrew not Yiddish for theirs.) I wonder if at some point, any of them will realise that investing in their distinctive language would be conducive to their long-term prospects of surviving the forces of assimilation.