I would add a slight addition to the "sell and get out of the business" aspect.

Since landlords, except in very rare circumstances, aren't going to tear the apartment down, selling won't reduce supply.  HOWEVER it will tend to result in the "good" landlords selling to the "bad" landlords who aim to make a return based off minimum costs, leading to the neglect and decay you mention.

The real long-term problem though isn't selling, but rather not building in the first place.  Ontario was a textbook example of this in the 1970's when rent control was first put in place: it DESTROYED the market for newly constructed rental units.