Generally yes, but I think it's a lot more nuanced and varied than that.

My guess is that there are a large set of genetic, environmental, and idiopathic contributors that converge to a loosely similar phenotype, but couldn't be treated effectively in the same way because they have different sets of causes.

I think we'll eventually get to a point where we can treat many kinds of dementia in the same way we treat infections/etc... But it'll take a while.