Poodledoc - In addition to the causes cited in your post, I can think of a number of reasons for the increasing prevalence.

I believe one cause yet to be addressed is the replacement of copper pipe in plumbing with plastics. The copper was naturally resistant to bacterial colonization, the plastics seem to attract the biofilm in which the mycobacteria can colonize. At some point I expect this will be more thoroughly studied.

Another is better and more thorough testing. I was treated for over 2 years, in 2 different states, for pneumonia, bronchitis & asthma (all of which I definitely had.) My sputum was frequently rapid-tested, but never cultured. Medication would occasionally ease the symptoms for a few weeks, then I was sick again. This describes how my mother lived for many years, until she ultimately died of CHF in her 80's. MAC was never diagnosed, but I am now quite certain she had it.

A sharp-eyed radiologist finally recognized signs of bronchiectasis in an x-ray and called my primary, who promptly ordered a CT scan and sent me to a pulmonologist for sputum culturing and the rest is history.

Finally, the "globalization" of soil-containing products - such as peat and potted plants, which we used to acquire locally, but are now shipped cross country may be exposing us to new variants of mycobacteria. Plants purchased from big box stores all over the country are largely raised, potted and shipped from Mississippi, Georgia and other places in the Southeast. So maybe we possess some tolerance for or immunity to the organism in the local soil, to which we have long been exposed, but not to a different strain shipped from another part of the country. For example, I gardened for over 40 years in Minnesota without issue, but when I began gardening in Texas, I acquired MAC.

Sue