I don't think scientists are necessarily assuming a single cause so much as that it's a moot point since we still haven't identified any causes for sure. If so

I don't think scientists are necessarily assuming a single cause so much as that it's a moot point since we still haven't identified any causes for sure. If some cases are caused by amyloid plaques and some aren't, for example, and we develop a treatment that cures the amyloid plaque cases but not the others, it will probably become extremely obvious that there are different causes at that point.