"if those carriers can be bothered erecting more cell towers"
there's the problem - any new entrant to the market can buy as much spectrum as they like, but they'll have to play catch-up to Helstra's decades of erections before they can actually use it. Gonna need some investors with an eye on the long game.
Or perhaps, they could just buy the spectrum, flog it on through a network of offshore shell companies and then flip it to big T for an obscene profit.