Being primary caregiver to a spouse who has dementia does substantially increase the caregiver's risk of cognitive decline due to caregiver burnout. [1]
I'm not sure if this is what you were trying to express.
[0] https://jnnp.bmj.com/content/89/3/231
[1] https://www.ovid.com/jnls/neurotodayonline/fulltext/10.1097/...
18 year study found unmarried more likely to not have dementia:
https://alz-journals.onlinelibrary.wiley.com/doi/10.1002/alz...
So either way, like how if ya live long enough some kind of cancer would get ya, so will mental illness
No beating physics
I think the easier way of looking at it is this: there is a Lancet commission that produces robust characterizations of the modifiable risk factors that lead to dementia. If you're in a marriage that promotes reducing those risk factors, you'll likely have a lower risk of dementia. Isolating one variate like this raises interesting questions, but isn't a "smoking gun'-- especially in this case since the authors themselves indicate this is a surprising result given other studies.
Seems just as reasonable but less verbose; we appear to get cancer and mental illness with old age regardless of social conditions around us. Physics and entropy win again regardless of the care we put into our semantics.
Regardless my original point was about "long term".
"Marriage staves of mental health" and "a single, long term marriage improves mental health outcomes" are two different things.
Could just be a tight knit quasi-nuclear family social circle is sufficient too but social desirability biases lead is to lean into marriage tradition.