As a type 2 diabetic I used a couple of different glucose monitors and got a lot of benefit from them. I gave one to a friend who I thought had diabetic symptoms. First I tried to get him to go to a doctor but he wouldn't. But he tried the CGM and found numbers well into the diabetic range. Then he immediately changed his diet and started treatment. Not sure but I may have committed a crime.

What was the potential harm that was greater than the reward?

Potential harm is always the same - misdiagnosis and/or mismanagement.

It’s probably very low in the context of CGM and diabetes as the potentially harmful treatments require prescriptions.

Device prescription requirements are usually due to product labelling and the manufacturers application. There are OTC fingerstick glucometers and CGMs approved.

Accidentally diagnosing someone is quite different from someone healthy trying to 'optimize' their health, whatever that means...

Strong disagree, diagnosing chronic dysfunction is essential to optimizing health. There's a big difference between an optimal blood glucose range and one that triggers insurance companies to authorize treatment. If you only pay attention to the later it could cost years of healthy life.

It's like, not being obese enough for your insurance company to pay for medical intervention doesn't mean that your weight is optimal enough to enjoy a long retirement.

Bad idea how? It's expensive but not dangerous. Some people find the results interesting, and serious athletes have had some good results using them to optimize diet and training.