You ask why the existing landline services are to be shut down.
It was part of the deal negotiated with Telstra and Optus.
In return for money Telstra agreed to separate their retail and wholesale arms and lease exchanges, pipes and ducts to NBN. There is a no compete clause where they can't use that infrastructure to compete with NBN, instead they retail NBN services.
When Abbott won office Turnbull renegotiated the deal so that more copper was retained in the network and also the Optus HFC network was deemed unfit for purpose so they started rolling out more of the "Glitter to the Turd" services, using shorter runs of copper to service premises.
So basically the contracts have non compete clauses.
If they want to compete against the NBN wholesale service they have to use wireless and other methods.
