I would suggest to you that there are many forms of COPD....and that your MIL more than likely had an infection/pneumonia...that caused excess secretions..and/or she had an inability to clear her own secretions.
Unfortunately I have treated many end-stage COPD'ers...and it can actually be a fairly "easy" way to go. The pt. often enough just can not ventilate effectively and their CO2 level climbs until they are basically unaware of what is happening....They just "go to sleep".
So, in my experience...it's not exactly as you have described.
FWIW-
I can only hope this was the case for my father. He told my mother it was ok to go home. She had been at the house maybe 10 minutes after a 5 minute drive there, when the hospital called. I think he knew.