This is just my own opinion but there definitely seems to be some indefinable point of no return.
The reason why I have this opinion is after being in the moderate category for decades, I sadly then found myself in a situation where I was forced into overexertion in order to make enough money to survive and pushing my body that way took me from being moderate to severe.
It then took me 5 long years in order to get my body back to moderate again. And that was five years where I no longer had to deal with the stress of work for a living or worry about my basic survival needs met.
While this is anecdotal, what I have gathered from that experience is that it is more than possible to reach a point in this illness if you became severe enough that it would take you more than the rest of your lifetime to recover from it.