Can old age really be ruled out, though?
We know that the Endless take the form of what they looked like during their happiest moment in life
This is shown through Wuk Lamat's mother figure and the side quests in Living Memory

Given her character, it would make sense then that how she appears before us is when she was most happy - traveling on the road with Gulool Ja Ja and the others.

My reason for thinking this is a comment she made when we first meet her, though admittedly I can't quite remember the full thing. She refered to herself as a young shetona maiden, or something along those lines which seems odd to me but could be nothing

It's tricky with Viera, because of their lifespan and I don't recall us seeing a visibly aged one as they retain their youth for the most part. I believe one thing that what I say would struggle to hold against is Iyaate, the blue-haired woman we meet in the MSQ who seems to be familiar with Erenville and his mother

I do like the idea that she was possibly killed due to her affiliation with the rebel group seeking to shut everything down though, and it's probably a much better explanation story-wise as to why she's no longer around but the other members seem to make no mention of it