One problem I see is that LLMs have a more nuanced... well, model of how words and their meanings relate to each other than a dead-tree thesaurus could ever present, what with its simplified "synonym" and "antonym" categories. Online versions try to give some similarity metrics, but don't get into the nuance. (It's not as if someone who takes either approach would want to spend the time reading and understanding that, anyway.)

> she could tell when students were using it to make their writing more fancy pants

I had two teachers who called us out on this, and actually coached us on our writing, and I remember them fondly. (They were also fans of in-class essaying.)

The others wanted to count big words.

In-class essays impossible? Pencil to paper?