I don't know on how much the subsidies from cable are but I know many people who would pay for HBO alone. Where I work some cord cutters pay other people to get HBOGO for them.
It will probably hurt the cable companies more than anything. Mine requires me to have expanded packages to get HBO. Those will be dropped. I'll take the smallest package they provide for the OTA TV and TBS. Where I live the reception sucks and I don't care to invest in a roof top antenna.