This is truly something we cannot know - there are so many other health conditions that can overtake a slow-growing MAC infection or stable or slowly worsening Bronchiectasis that we never know if these shorten our lives or not. This is true of many conditions.
For example, my Dad had both heart and kidney disease - quadruple bypass surgery on his heart, stents in his renal arteries. Suddenly, he was diagnosed with Stage 4 liver cancer, and we lost him just 4 months later.