Risk vs Reward!, page-92

Gladstone cost 508 million as was finished in 1976.
https://en.wikipedia.org/wiki/Gladstone_Power_Station
It is on borrowed time as it is 45 years old. It would be untenable to build the same thing today.
.https://www.energynetworks.com.au/news/energy-insider/the-demise-of-coal/

I don't think the people who live near the coal plants in the world are super happy about the health effects either.

.https://www.climatecouncil.org.au/killer-coal-just-how-bad-are-the-health-effects-of-coal/#:~:text=In%20the%20United%20States%2C%2050%2C000,stroke%20and%20chronic%20respiratory%20diseases).

The cost to build a new coal power plant is not cost-effective given the downward pressure that renewables are putting on electricity prices..
New coal power plants are not a fiscally reasonable venture for Australian companies no matter how much the government and the coal lobbyists wishes they were.

This is what is likely to happen in generation over the next 20 years..

AGL is basing its strategy on science and economic modelling by experts. An engineer (I am an engineer too by the way) who respects science and mathematics as tools of the trade I expect would respect the numbers. AGL has a vested interest in keeping the company going and would be using credible sources as well as their own internal research and economics folk to work this out.
I am not a fan of renewables based on principle, I am a fan based on the innate efficiency of taking energy as directly as possible from our big nuclear reactor called the sun and convertng into a usable state as efficiently as possible with out stuffing up our air, water or food. We can then use that energy for coverting materials into all the things that we need and want without turning the planet into an oven or swimming pool at the lowest possible cost.
Lower cost of energy means we can covert materials and move heat for less and this improves our GDP which in turn improves the living standards of the population. This will bring more jobs and prosperity to more people than building coal mines and power plant with less negative indirect impacts.
The transition over time to renewable sources of energy is happening so any competent management would be planning for that. AGL has started the process for managing that change. There is obviously going to be a hit to income in that process but better that than hopelessly sitting on aging assets with worsening margins and just peter out?