We need both. Air transport produces a ridiculous amount of greenhouse gas emissions, and nobody is ever going to replace a plane with a train at low speed train speeds. With high speed rail and properly-priced externalities, almost all Melbourne-Sydney trips (pre-COVID, one of the busiest flight routes in the world) as well as stop-offs in Canberra could easily be done by train. You probably wouldn’t replace as many Brisbane-Sydney trips—if externalities were appropriately priced-in, you’d most likely see leisure and family trips done via train but business trips continue to be flights.

You’re right that we also need more frequency, but that almost automatically comes with the high speed upgrade. High speed rail replaces flights in the sub-1000 km range. Low speed rail is its own category that simply doesn’t compete in the same space as flights, which is why the low frequency unfortunately makes some sense and increasing it wouldn’t help. But when you’re competing with flights, the volume of travel, and therefore the frequency of trips, goes way up.