> The fact that these are still very much niche interests after many generations indicates that the social indoctrination processes needed to maintain such cultures are not scalable.

In 2023, Israel contained over 1.28 million ultra-Orthodox Jews, 13.5% of Israel’s population. It is estimated that by the end of this decade, it will be over 16%. [0] By shortly after 2065, it is estimated that 50% of Israeli children will be Haredi. [1] I think this is counterevidence to your claim - the social indoctrination processes necessary to maintain Israeli ultra-Orthodox Judaism have already scaled to over 1 million people, and are projected to scale much further than that

[0] https://www.timesofisrael.com/haredim-are-fastest-growing-po...

[1] https://archive.md/lYYgK

So in % terms not even a rounding error of the global population by 2085. These religious communities rely on both cult-like indoctrination and quasi-incestuous family networking in tandem to survive. We are not going to see Amish or ultraorthodox Jewish populations taking over the world.

Don't the Haredi refuse to enlist? Doesn't seem sustainable...

Refusal of enlistment is not essential to the Haredi lifestyle, and so it seems plausible their resistance to that might gradually weaken

There is already a minority of them, the Hardal (Nationalist Haredi), who are happy to enlist, but whose birth rates, retention rates, and general lifestyle, are similar to the Haredi mainstream