Since I spend about 80% of my time bedbound (physically, I'm on the more severe end of the spectrum), I feel a little offended at the idea that there's anything "luxurious" about my situation. But I'm figuring you didn't mean it that way,
@deboruth.
What you say does point to the idea that different people might have different illness mechanisms.
So here's my thought. There's a group of us where the primary problem is to do with inflammation. This inflammation is stimulated by exertion. When stimulated we feel awfully sick, flu-like, with lots of burning and aching and headaches. These effects are often delayed (for example, they may peak the day after the exertion). So our main goal is to keep these symptoms to a minimum - hence, avoidance of PEM. I'm almost certainly in this group.
Then there's another group where the problem is primarily to do with energy production. People in this group talk in terms of a battery - once the energy's used up, that's it.
@deboruth, your description suggests you might be in this group.
I do occasionally get what you describe as collapse if I push too hard, but its more like burning, screaming pain than a feeling that I've run out of energy. More often though, I push through with no immediate symptoms, not till the next day. But then the price may be severe malasie, burning, headaches, so bad I might not be able to move my head for days. Its all very episodic so its easy to send yourself to bed for a fortnight, then feel a little better one day, do a few small things and then start the cycle all over again.