The "disposable" electronics in these are also extremely interesting. They're cost-reducing the hell out of these devices. Even in the last few months, one vendor replaced a cheap chip (TI RF430) with even cheaper custom silicon. They are not standing still!
According to Wikipedia, first CGM was approved in 1999. That may be recent for medical devices I guess, but it still seems like they're charging a lot for 20+ year old tech...
CGMs have improved a ton in the past 20 years, becoming less intrusive and easier to use.
We pay a lot more than $150 for all sorts of tech that is far more than 20 years old. But if you could make it for $20 or $30/month, that would be a serious innovation and perhaps you should try to start an advanced medical device company.