If your body is in good shape, stopping exercise won't make you deteriorate that quickly. What I wonder is, will people get in good shape in the first place.
What I mean is as someone with lots of experience, I don't care about me not learning about the basics anymore as much as someone in their 20s and 30s maybe should.
I think this is backwards.
Not sure what you mean by quickly. Back when I was in racing shape, if I stopped my training plan for as little as two weeks, (probably less actually, but I'm being conservative here) I would have a measurable drop in fitness.
Now, as someone who regularly walks the dog and bikes to work, I've got "less to lose" and probably wouldn't deteriorate as much.