I agree but that doesn't mean they won't use her again, it just means that the writers treat characters like toys to disregard on a merry go around where they only pick them up again when they come back and the plot calls for it.
Glynda is a fan favorite side character with more relation to the cast than others and RT has shown repeatedly they lack the balls to make big story decisions like killing one offscreen, I can understand Vale because as someone would put it.
I have severe doubts that she would never have come up again. Yes, they had problems with her voice actress in the past, but given what happened with Qrow's RT has shown that they aren't above simply replacing a voice actor and it's not like Glynda had many iconic lines for any would be voice aspect to live up to anyways.
Most likely the entire thing with Glynda being MIA along with her cameo appearance in V8 was to build up shock value of where people endlessly hype up her return, RT brings her back, she does one thing of badassery or irrelevance and is then summarily killed off while again none of the main cast dies.
In short, I agree on almost all points but at the end of the day saying we wouldn't see Glynda again is supposition that we will never have proven or disproven given what has happened to RT and RWBY as a whole.
And at the end of the day does it matter? If she lives, she lives and dies by shitty writing and if she died in Vale she again dies to shitty writing, it all proves RT's incompetence all the same in the writing department.