Before there was the ability to provide respiratory support the death rate from MG was about 30%.
In patients who had significant respiratory symptoms it was nearly 100%.
I believe this is also the source of the scary name of myasthenic crisis.
At that time it made no difference if you recognized early signs of it, because there was nothing you could do anyhow. Physicians could just hope that their specific patients belonged to the 70% who would do reasonably well.
The word " crisis" was (I believe) coined by Hippocrates to describe pneumonia before the era of antibiotics. All a physician could do was sit next to the patient, hold their hand and hope that they will recover from it.

This changed dramatically to less than 5%, once there was proper respiratory support using mechanical ventilation. (it also led to many needless intubations because of the fear of rapid respiratory deterioration).

Assuming that MG remained the same disease, it is reasonable to assume that patients with significant respiratory symptoms, who are left untreated would die just like they did before.

This also means that if a patient with MG dies because of respiratory failure, it is quite likely that he/she did not receive proper treatment which could have saved their lives.

This doesn't mean that if a patient with significant respiratory symptoms doesn't have severe and rapid deterioration, it is not serious and can't be due to his illness. The respiratory symptoms of MG are fluctuative (just like all other MG symptoms) and patients can have continuous worsening and improvement. They can be on the verge of requiring respiratory support and then recovering with rest and medications. There is no need to wait for that one time in which they will have more persistent respiratory failure.