You're being downvoted without anyone responding, which seems unfair to me. I disagree with you, but your point is valid (if more certainly phrased than I would have gone with).
You're right. As a species, we are wrong nearly all the time. For all the things we know, there are dozens of theories that were contemporaries or predecessors of the correct answer as we understand it now. You are entirely ignoring that our understanding does generally get progressively better until we fully understand the subject in question.
> At one point we were killing cats to curtail the black plague, when in fact it was the fleas that were the cause.
You're right, the cats were not the direct cause, and doing so may have even increased the spread of the Black Plague by allowing rats to spread more quickly.
On the other hand, blaming the cats is much closer to the actual answer than what we would have assumed 500 years before that. Namely that some deity was angry at us, and the only way to stop it was to offer gifts to that deity. And treating it through "humors" was a bad approach, but again, much better than guessing that the person had been cursed by a deity and that their survival hinged upon making good with that deity.
Our understanding is progressive, not binary. Our correctness is binary (i.e. we're either right, or we're not), but our understanding usually comes in steps where we slowly get closer and closer to the right answer. For fasting and dementia, who knows? We might be right, and we're at the end of the tunnel. Or we might be wrong, but hopefully one step closer to figuring out what's actually going on.
> We just don't know.
This part I really don't like. There's a difference between "don't know" and "can't know". Assuming you place some level of inherent value on knowledge, it would be optimal if we did know all the things that can be known. Even if you don't place inherent value on knowledge, some knowledge is useful, and surely we should seek to know those things? It's just difficult to determine whether a particular piece of knowledge has use or not. The Planck constant doesn't strike me as particular useful, but I'm almost positive it is to other people.
> And it's fine not to know.
I also heavily disagree with this part. People are, quite literally, dying in a way that is unimaginable to most of us. Getting dementia is a terrifying thing; it's one of the few things in humanity that can make you entirely lose "you". I've lost several relatives to dementia, and it's heartbreaking to watch. Just alternating periods of dementia where they're practically not there, and periods of clarity where they're terrified about what's happening to them. Death truly is a blessing to someone in advanced stages of dementia.
"We just don't know. And it's fine not to know." reads like some late-stage depression that has manifested itself into nihilism. There are tons of corollaries that are probably a lot less comfortable, but with the same implications. "Famine has always existed. And it's fine to not help.", "Racism has always existed. And it's fine to be racist.", "We've tried to stop child abuse and it hasn't worked, so it's fine to stop trying.". Just because something is currently true does not mean it will always be true, nor does it mean that trying to change it is futile.
I'm not particularly religious, but your response does remind me of the beginning of the serenity prayer:
> God grant me the serenity
> To accept the things I cannot change;
> Courage to change the things I can;
> And wisdom to know the difference.
The most important part of that to me is the wisdom to know the difference. I fundamentally believe that dementia is something we can change, given enough time and resources. That makes it something worthwhile to pursue.