On those websites, a listing that old is either the owner just trying to see what they could get or it’s a bad property (either economically or physically).

I don't know what a good vacancy rate is as opposed to a bad one.. but if your managing 2k to 10k houses I suspect your always have some manner of vacancy.. the main take away for me is you simply cannot buy property in the "cash flow markets" were your buying in the lower third of the economics of a given MSA.. those are just tough for whatever reason history has proved this.. most of the buyers have only them selves to blame here.. they are all mesmerized about COC returns and take risk they just don't understand.. then they blame the tenant and the asset.. when in fact its their own choices that lead to the inevitable in low value asset or bottom third in price point in a MSA..

The opioid crisis has hit this country hard, and I suspect that is part of what is happening here as previously solid tenants flaked out, and another left due to violent crime.

As @Chris Clothier and @Curt Smith have pointed out, there is a lot of good data on economic and demographic trends readily available.

https://www.investopedia.com/articles/economics/09/1970s-great-inflation.asp

I just designed very economical 2b/1/b 6-plexs based on a 30x30 (900 sf) footprint for a developer.

And when the crash hit you weren't in a crisis mode. 

(we're talking about economics, there's always an other hand!)

Also, through normal economic cycles—I would think a stock market correction may be naturally occurring in 12-18 months so I can accept a modest returnI only have exterior pictures of the property which shows it as “fine/livable”.

With more relaxed rules from July 2018, it makes a lot more economic sense to invest into building an ADU: http://www.sanjoseca.gov/DocumentCenter/View/78534