The NBN was gold for Telstra
They paid through the nose for all that copper and ducting just before it became obsolete. (Not because it stopped working, but because Mobile data replaced the need for it.)
The growth in mobile phones has finished. Everybody already has a phone with lots of data.
4/5G will enable Telstra to move many people from the NBN to Mobile. (Most households do not need terabytes of data.) But then the party will be over. Optus and even Vodafone are clipping at the heals of Telstra -- no easy monopoly for Mobile.
This has happened before -- cars, TVs, plumbing. A huge growth when the technology became available, then a flattening of the demand. Now that people (almost) have all the data they need, this game is out of its growth phase.
I see a long future for Telstra. But not one of spectacular growth.