According to the ratings , the NRL is the most watched sport in Australia. So the interest level is there.
Clubs such as Newcastle , Easts, Nth Qld etc. all want teams. As do Perth, PNG ,C Coast etc. And they all want to play in the number 1 game but not all can fit. So If the lower tier is attracting a grant from from appearances on tv/streaming . And assuming the networks pay as much or similar for 6 top quality matches in the top tier each week there would also be a little extra there. P/R play offs could attract almost as much interest as the Grand Final.
There would be a lot of people like myself who would gladly pay to watch my team , particularly if they were a chance of being promoted . If teams were close to being promoted , sponsors, benefactors even governments would come out of the woodwork to back there team .
And as has already been stated, if your playing badly you don't deserve to be in the top tier. And if someone with strategic importance like Brisbane were relegated , there backers and supporters would soon have them back in the top tier once they started playing well.
It would be sink or swim, and if you sunk it would be nobody but you and your clubs fault.
Remember there supporters are dormant not dead!