I was a fairly active person, but not really one that went to the gym. I am not sure if it would have made a difference health wise for me, but it sure as heck would not have added to the health problems. Active life does not necessarily mean a healthy life.
Diet, exercise and sleep all make for a much better life than an active life. We put off sleep to enjoy time with friends and family, we put off healthy food choice for convenience of work and gatherings, and we put off exorcise so we have time for work, sleep and friends and family. In the end it all comes down to being able to better manage the time we have in a day, which I never did learn very well, and still do not have a handle on even though I have all the time in the world being retired.
Yeah, I have no idea if gym helps everyone, but being active (on average) will yield better results than not. I was very active too - doesn't stop illness :)
That balance between all the things we are "meant" to do, is difficult for me at least. Some seem to manage their time and resources far better than I though.
That time management is a tough nut to crack for me, still have not managed since leaving home at 18, and that was a long long time ago. 40 ++ years ago.