I disagree. You'll still be missing out on a lot.
It depends upon your concept of what it means to be ‘healthy and fit’.
Yeah, it does depend on the definition of "healthy and fit" which aren't very specific terms. I'm putting it in context... I believe S&S is a great way to check the boxes for exercise as it relates to health promotion, risk reduction, and function retention. You can live a full and complete life if this is all you pursue. Just like if all you do is the elliptical and Nautilus circuits. Some people don't want "all the skills", "all the strength", "all the abilities." They just want to stave off diseases of aging and develop some full-body strength and athleticism. S&S does this. (Caveat... if you don't
do it well, it doesn't
work well. Focus, intent, getting instruction, reading the book -- all help to actually achieve the benefits.)
However, I agree with
@renegadenate that you would be missing out on a lot. There are many more aspects available to train. Speaking for myself, I've really enjoyed going beyond S&S to SFG I and II, barbell and bodyweight strength, weightlifting, etc. But I, like everyone else who spends time on this forum, enjoy going deeper into these topics. We only have to look around us to know that's not the norm.
I could make some analogies... If I always eat my same favorite 12 healthy meals, aren't I missing out on a lot? Yes, but it works for what I need and keeps me healthy and fed. If I only read three books in a year, aren't I missing out on a lot? Yes! But 3 books are better than none. If I know my basic family history but have not researched older generations and ancestors, aren't I missing out on a lot? Certainly. Someday I might get a chance to discover more. If I attend just one of my kid's or grandkid's baseball games, aren't I missing out on a lot that I could get if I went to every game? Of course. If I do a bit of home improvement work, am I missing out on how much more I COULD do? Yes, but we all have that amount of effort and time that we choose to put into something like that. If I do some basic saving and investing of money am I missing out on how much more I could earn if I really delved deep into it? Probably so. But most people just want to do the basic required commitment and effort in this area and let time and the market take care of the rest. Etc....