In the long run though, how much of a threat are they even if we assume a best case scenario for each one being converted?
They have almost no financial safety-net beyond things such as welfare. (family and community strained by number of children and low-income)
College is unattainable, and being home-schooled it is questionable whether or not they even have a high-school education.
The most force that they have is the vote, and possibly running for local offices. Even then, they're going to be controlled by conservative groups who view them as a vote and will undermine the social programs that they use.
"Vote with their feet?" and apply pressure on non-christian companies? Laughable. Global economy, even assuming a monolithic 0.5 billion group of these people with perfect grassroots communication it's not a real issue.
In the end, it is an unsustainable strategy and increases the risk of deviance. (i.e. kids seeing 'evil secular families' with more resources)
If anything I want this to be embraced, as it'd collapse within two generations and in the end disillusion people even more to organized religion.