If social consequences were sufficient, we wouldn't have invented this whole legal system in the first place.
Social consequences are not good for everything, but there are a lot of areas when there are better than economic approach. The problem is that economic incentives are much stronger than social, so the latter can be easily destroyed by the former. The classic example would be the fact that you absolutely, positively don't want to pay people for donating blood - it immediately leads to contamination of blood banks, as people replace the sense of duty with an opportunity for free cash, and thus stop caring about things like not donating blood when they are sick.
Since social incentives are fragile, you want to defend them in the areas they work better than economic ones - hence the opposition to AirBnB.
Exactly this! The consequence of letting people abuse trust on massive scale is more complicated contracts, which is a worse outcome than if everyone just agreed to act in good faith.
Rental agreements have already ballooned into legal tomes no first-time lessee could ever understand. If they now have to spell out penalty for a variety of different breaches, they will become much worse (why not a single, higher penalty? That will get abused to extort tenants who commit minor violations...)
IMO it should not be about contracts. You should enter agreements in good faith and there should be social consequences for doing otherwise (reputation ruined, ... etc.). Be excellent to each other.