I agree with you. But it's clear that "begging the question" is going the way of "literally," and there's (sadly) nothing we can do about it.

I suppose some time in the future, someone will invent a new phrase meaning "assuming your conclusion".

At what point did dictionaries providing descriptive views of the English languages turn into a prescriptive one that emboldens people to just point to repeated wrong usage rather than admit they were wrong?

assuming your conclusion, why would we need a new phrase?

Well, I for one won't be party to it. I think informing everyone I can is my drop in the bucket in the fight against the incorrect usage of words. :-)