If you think about the service industry needed to support the retired population, I suspect you would find the following:
Health Industry (hospital, pharmacy, therapy, nursing, home health and hospice, etc).
Finance & Insurance (wealth management, insurance, banking, investment, real estate, reverse mortgages 😣, etc)
Tourism (Hospitality related, food services, laundry/cleaning, activities)

I’ve lived there for 30 years. Outside of construction, doctors, and attorneys, most had a job or both in these areas.

There are some changes coming to SG. More manufacturing, more tech jobs, and a lot of people working from SG and commuting to California and Vegas. The airport has helped a ton.