There is one thing that swimming isn't the best for, and that's preserving or increasing bone density. I didn't post links, but googling "Swimming and bone density" will put plenty at your fingertips.
Beyond that ... it's exercise, it's supportive, it's non-impact. There's no concerns about being a sweaty mess.

It moves your body in ways that make it functionally stronger and more flexible for everyday life.
I think your instructor probably got ahold of one article, read the headline, and felt like an expert. Unless she's ready to go into physiological details of exactly why she thinks her statement is true, and to expound on a large-scale, long term study, it might be better to nod politely and continue to keep doing what feels good. One of the greatest lessons I've learned here at 3FC is that each one of us has our own path.
I've been in and around competitive swimming for almost 40 years, and personally know a lot of folks who rediscovered health and fitness through swimming, even well up into their 70's. If you love it, you'll keep doing it, and your body will respond positively. Happy splashing!