The answer is right there in your citation:
In the era before corticosteroids were used for treatment, approximately one-third of patients improved spontaneously, one-third became worse, and one-third died of the disease.
Why do you think those people died? because they had a stable disease?
I have not yet heard of someone dying of a stable disease.
Every autoimmune disease has a variable and many times unpredictable course.
For example: I have patients with immune thrombocytopenia who had a severe disease, requiring hospitalization and aggressive treatment, who after a few months fully recovered. I have others who had a very mild illness, never requiring treatment for many years, and yet others who had a very rocky course, achieved remission for a long period and then had recurrence of their illness, which was now much harder to control.
MG as opposed to thrombocytopenia is a disease that affects your function, and you eventually adjust to it. This may give the illusion of a stable or "burnt out" disease.
6 years ago, when I woke up in the morning hardly able to move, talk or breath I ended up in the ICU and told I was pre-intubation.
Now, when I wake up in the morning like that, I take my meds, and lie down with my respirator for a while. I then get up, and go to work, return home early so I can rest more. Use my respirator on and off for a few days, until I gradually get back to my previous base-line.
So, is my illness stable, burnt-out or did I just learn to live with it better?