With a population that is constantly declining for the past 50 years and that is older than the national average, you should not be surprise by the number of abandonne houses in Onomichi as those are two of the major factors that explain why the buildings are abandoned. What also does not help is the geography of the city, where many houses have been build on the hill side where in many cases re-building could be prohibited under the current law. Unfortunately, that mean that the small streets of Onomichi are dotted with abandoned houses.
Luckily, there is group of people who dedicate their time to promote the renovation of these old building, either for residential or commercial use. This is the case of Miharashi-tei.
Build in 1921, the building was out of use for nearly 30 years. The renovation on the building started in 2015 and completed in spring of 2016. It is now a guesthouse with dormitory and private rooms, but it's also a cafe/bar. I only visited the cafe where I ordered a delicious orange juice and a set of cookie. The inside of the cafe is really cute and the view from the window is just fantastic, it was even better as I was there right during cherry blossom season. I would personally recommend going at least to the cafe if you are interested in sustainable economy and in renovating abandoned buildings. Just check the schedule as they have different hours depending on the day of the week.
If by chance you decide to stay there for the night, just be aware that the access require some walking and as they say, there is about 360 steps to go up the hill from the station. There is also no access by car, so also no access via taxi, but you could park in the Senjoki park parking lot. They also recommend parking your bicycle at the bottom of the hill if you cycle to Onomichi. If you have large luggage, you might want to leave them in the coin locker at the station or store them at the Kuroneko Yamato office, there is one by the port and one along the shopping street.
I would for sure recommend to go, at least to the cafe, but if you want to stay for the night in a restored building with easier access, there is actually two other hostel/guesthouse with a cafe space located along the shopping arcade, Anago no Nedoko and Yado Curly.