from my perspective, the bike has been neglected and then abandoned ,and its unlikely that the owner will return and then if they do invest the time and effort in restoring and maintaining her.
Im not advocating for you to steal bikes, but to save or rescue a bike is another matter, and in my mind over rides any moral objection to theft which is based on our own fear and feelings when or if a bike we own is stolen.
You could make a deal with your self that if you are ever stopped in the street and someone says thats my bike, you give it back without objection, saying that you was only looking after it for them,
if your 100% sure that the bike has been left as long as you say, then go for it, I amazed though, even in the most respectable parts of Birmingham, you could not leave a bike chained up day in and day out with out at least bits going missing