I don't believe there's any reliable evidence that getting worse and worse in the early years is the natural course of the illness.
When ME/CFS appears to be triggered by a viral infection, you either get the instant onset ME/CFS, in which the full severity of the disease hits you within days of contracting the infection; or the gradual onset ME/CFS, in which the full severity of the disease slowly appears over months or even years. So in the viral onset, this disease either hits you all at once like a ton of bricks, or you more slowly ramp into the disease.
So I am just pointing out that during any period in which you are ramping into ME/CFS, this would not be a good time to test the effects of exercise.
Any anecdotal evidence of worsening in the early years would have to be considered in the light of the fact that most of us either didn't know what we had in the early years, or didn't know how to manage it properly, and so were constantly overdoing. So the experience of worsening for the first few years could just as easily be attributed to deterioration due to overdoing as to it being the natural course of the illness.
It could be attributed to overdoing it, or it could be the natural disease progression. Unfortunately we do not know which.
I mentioned above that I think the most reliable way of seeing if regular exercise (that pushed you into PEM / crashes) could permanently worsen ME/CFS would be from a patient who was symptomatically stable for several years, and then they started pushing their exercise envelop a bit.
In Dr Chia's interferon treatment research, in patients who went into remission as a result of the treatment, it was strenuous exercise that caused a catastrophic relapse into ME/CFS, and a concomitant return of the enteroviral infection that each patient had. So here is good evidence that exercise can be a bad thing went it comes to enterovirus-associated ME/CFS.
However, as for evidence that a stable ME/CFS patient might get worse if they engage in regular exercise / PEM / recovery cycles that are not too strenuous, I have not seen any.
Maes and Twisk did publish a
paper in which they stated:
this review shows that exertion and thus GET most likely have a negative impact on many ME/CFS patients. Exertion induces post-exertional malaise with a decreased physical performan-ce/aerobic capacity, increased muscoskeletal pain, neurocognitive impairment, "fatigue", and weakness, and a long lasting "recovery" time.
But I don't think there was any hard evidence for this negative impact (the full paper is not available for free).