Cognitive decline and dementia are multifactorial, and with alcohol consumption there are trade-offs between neuronal death, potential improvements in vascular function, and other things known and as yet unknown.
Here's an interesting recent finding from a study of 20K BioBank participants: "Alcohol consumption above 7 units weekly associated with higher brain iron. Iron accumulation represents a potential mechanism for alcohol-related cognitive decline."
https://journals.plos.org/plosmedicine/article?id=10.1371/jo...