Places with public school vouchers as law allow said vouchers to be used at 100% religious schools.

This:

     bolsters shitty religious propaganda
     dumbs down the students receiving substandard education
     props us churches
     drains money out of the public schools
     requires that I pay a part of my taxes to an unaccountable religious organization. 

I'm sure a few google searches would show more comprehensively why this is a bad idea.

> Places with public school vouchers as law allow said vouchers to be used at 100% religious schools

This isn't a requirement for vouchers. You can simply state that religious schools aren't eligible.

What is the realistic likelyhood of this being mandated, given how many religious private schools there are and how much money they have to lobby with?

> What is the realistic likelyhood of this being mandated

Just under a third of Californian's aren't religious [1]. Nobody is proposing to ban religious schools.

If you can't swing it, make the choice local. Each county or whatever can vote on whether to permit religious vouchers. The Bay Area will ban it. The Central Coast won't. This remains better than the status quo. [1] https://www.pewresearch.org/religion/religious-landscape-stu...

This is incorrect as of last year: https://en.wikipedia.org/wiki/Carson_v._Makin

There is a powerful political and legal movement behind the idea that state-funded religious indoctrination is not only permitted, but required.

Oh, so in a voucher system I would've had to choose between receiving a worthwhile education and being queer. Got it.