You make it sound like there is only one way that yields can improve.
Situation: Negative gearing removed.
Belbo says: All landlords increase their rents in unison.
I would suggest a more likely outcome is that most property investors try and flee the market. With an already elevated amount of housing stock for sale they will be forced to leap frog eachother and accept lower prices, falling prices which will in turn increase the yields (for new buyers) without the need for rents to rise.
With 1 in 11 tax payers (e.g. a majority of property investors) owning a negatively geared property I would liken this group not to a cartel (which indicates a small, sophisticated and powerful group), but to a flock of lemmings who will exit the market should anything disrupt their precious tax reducing property "investment".