I DO have direct experience. I know how dementia progresses from various elderly relatives, some who went downhill with it, some who didn't. My anecdotal evidence of a very small sample is that the fit, healthy and active ones succumbed much later in life, whereas the ones who weren't fit, healthy and active succumbed at younger ages. That's my experience!
I also understand statistics, probability and medical research.
From my own direct experience of MIL, I can observe first hand that she's a lot more "with it" when we've taken her out for a walk, shopping or whatever - once we're back, she's more chatty, remembers more things, and when we leave, she's not constantly on the phone asking inane questions. On days when she's not been out, and just sat on her sofa, she's like a vegetable, can barely talk, can't even remember our names and once we leave, she's constantly phoning asking the same questions over and over again.
We try to take her out as much as possible to get her blood flowing - in her case, it makes a massive difference, we can see it. Until covid, we used to take her with us on holidays, UK and abroad. Granted the dementia wasn't so bad, but it was still apparent how more engaged and "with it" she was on active days, as we were literally living with her 24-7 for 1 or 2 weeks. She was remarkably "normal" on "big" days of lots of activity, like the arrival and departure days when we did a lot of walking around airport terminals, etc., or in full day events like theme parks!
Unfortunately, OH's sister is like some posters here who are in denial about that, as she's just happy to sit and watch tv and drink tea when she goes to see MIL, with the inevitable result that she can't even remember sister has been, even literally minutes after she's gone. Whereas when we take her out, she can remember going out, at least for the rest of that day.