Your reply that there is "decent evidence" that glocosamine chondroitin helps prevent arthritis in the knees is helpful in itself. "Decent evidence" sort of says to me, keep taking it, there's a good chance it does help.
Your advice about the Folic acid, vitamin B12, vitamin D, magnesium, and fish oil, ie, that they are "believed to preserve and improve brain health", has rather left me on the same fence I was on before. "Decent evidence" for glucosamine was good! "Believed to" , however, begs the question by whom? The company that makes them? Medical reports? In short, is there no real "decent evidence", as there is with glucosamine, one way or another? In even shorter, would doctors, themselves, on average, consider the probabilities that they actually help strong enough to take them?