I agree that it would be worth trying to research this a bit. I do not have the time just at the moment but might later.
I had a quick look at reviews of exercise studies in various illnesses.
It seems that there are plenty of studies that did find improvements on objective outcomes. Reviews have reported decreases in
hospitalization rates in patients with heart disease, reduced
sick leave in low back pain, improvements in
tidal volume in COPD and
oxygen desaturation index in
apnea, decreased
insulin resistance in diabetes type 2, muscle power in power in patients with
MS or
spinal cord injury, neurocognitive function in
Parkinson's disease and balance, mobility, and walking endurance in
dementia. There is even a review that reported strong evidence that exercise improved key biomarkers in
atherosclerosis such as TNF-a, IL-6, CRP, VEGF etc.
There are also reviews that report objective improvements in exercise capacity (often measured by VO2, muscle strength, 6 min walking tests etc.) in various illnesses such as
renal disease,
MS,
fibromyalgia,
breast cancer,
schizophrenia,
Down Syndrome,
Angina,
Prader Willie syndrome and probably many more. I didn't have the time/energy to check the reliability of the conclusions in these reviews. Many are based on approximately 10 studies, often small ones (much like the situation in ME/CFS) and with some inconsistent results. But even so, it's clear that there are some indications that in these diseases that exercise improves objective outcomes (unlike the situation in ME/CFS).
The fact that there are hardly any objective improvements in ME/CFS isn't normal. Peter White argued that deconditioning might explain ME/CFS symptoms and that it forms the rationale for GET. Regarding CBT, Wessely said that "the primary aim of treatment is to restore activity." So it shouldn't be too hard to find objective outcomes to test their theories and the effectiveness of their treatments. The data indicate that GET and CBT do not improve physical fitness or activity levels, and it's only afterward that they've argued these objective outcomes are unfit for purpose. I don't think there are any excuses. If exercise improves fitness in all these other diseases, but not ME/CFS - the illness they describe as characterized by kinesiophobia, exercise avoidance and deconditioning, then it should be clear to every honest observer that something is wrong with these assumptions or treatments.