I think both Whathome and Calina are right:With the current situation, private vendors are subsidising developers. Developers can currently charge a stamp-duty premium as their sales are exempt.
SD-exemption on new homes was supposed to stimulate the building of new homes targetted at FTBs. That is no longer required, so either SD all FTB homes or none.
But it still doesn't address the fundamental issue that it is the increasing supply of cash/debt that has made prices reach these crazy levels where an above average earner cannot afford a FTB house.