Don't worry, the NBN won't need to upgrade the back end. You'll be getting FTTC. That means you'll have fibre running past your house.
5G will never match the cost/data that NBN will be able to provide. All the telcos rolling it out will need to get back their return on investment, so you'll never see large amounts of data on plans, just like we do now with 4G.
As for me, in my old house, we were a couple hundred metres from the node. On a 50/10 plan we got around 44/8. I thought that wasn't too bad for the crap copper. We're now renting and I think we're a fair way from the node. Wouldn't actually know which one we're connected to. We get 27/6 on the same 50/10 plan here. Stupid FTTN and the crap copper in the ground.
Looking forward to building our new house soonish that is in a brand new greenfield estate with FTTP rolled out ready to connect up to new houses
FTTP is the only thing that should have ever been rolled out for this project. They should have stuck to that and future-proofed everyone's telecommunication needs.
The current record of throughput for fibre is 159Tb/s over a distance of 1045kms.
https://phys.org/news/2018-04-fiber-transmission.html