Simply not enough margin to carry an old decaying assets and tenants that don't care.

You are making the case that requiring this would cause the property to continue to decay since no one will be able to take on the task that the City is trying to require.You'll probably have to end up getting them both to be on board with keeping the nonconforming use, but if you can get one then they can become your advocate for what you want to do.

In the meantime, start making good financial decisions to counteract the negative. 

One owner was in a nursing home for over 10 years, the place noticeably decayed and scammers managed to get a $200K HELOC on it, cashed out and skipped town.

This seems kind of counteractive for them to get paid to try to fill the room and then get paid if they don't fill the room and I get no rent from it, while it is 1 less room with a tenant that they have to manage.  

That could counteract purchasing a more moderately-priced property.

Urethane/AO Finish on Carbonized Products• Aluminum oxide detailing counteracts the wood softening from the carbonization process.

Everything is either worn out, or deteriorated, even the front door is broken, and decayed.

That being said, they also decay over time and do need to be replaced. 

One problem you may encounter as with a condo the maintenance fees increase so the bank may want a higher cushion than just $50 a month to counteract when the fees go up..