Exactly, the CBD is no more and hasnt been for years,it shouldnt now be the major interchange for bus services throughout Newcastle
The hub of Newcastle should now be Broadmeadow.
It is a pointless exercise to have to train it to the end of the line at Newcastle to make a connection to buses and then head back out again if you want to go anywhere in Newcastle by public transport
The flaw in your argument is that there is no need to truncate the rail line in order to move the bus interchange.
There is still reasonable commuter traffic to and from the CBD and environs. At peak times (i.e., when it counts from a congestion point of view) this class of traffic absolutely dominates service use - it might even be the biggest single class of passengers across all times. On the other hand, I'd expect the number of people interchanging from bus to rail to be small. Your specific use example would be an infinitesimal part of the use of the system.
An interchange at Broadmeadow is a distinct disadvantage for this direct commuter group. Estimates of the likely delay due to interchange have been modelled in one of the studies conducted by NSW Transport and were of the order of 15 minutes, from memory, with time savings for other passengers being nominal. That increased delay was expected to have a noticeable effect on rail patronage - I recall the expected decline in patronage from a requirement to interchange was of the same order as the potential growth in patronage from any one of the public transport improvement options being considered in the report. The movement of the interchange, in isolation, was not expected to grow total public transport use - if it did, then we would be having a different discussion!
(Note the use of "from memory" and "I recall" - I don't have the report handy at the moment to check details.)
If you relocated those commuter jobs to Broadmeadow, then the situation would be quite different. But I very much doubt that features in any plan to "revitalise" the CBD. Perhaps it should. Relocation of the rail yards, which would be well and truely on the cards if the Hexham bypass was built, would give you a very large parcel of land that you could use to build a new, well connected town centre from scratch.
For your specific location (Lake Macquarie) the intended point of interchange in the future for services to Sydney will be Glendale. That interchange has budget funding and appears likely to progress - but I agree with mm42 that the other Save our Rail wish list projects for Newcastle are delusional, that said I also think mm42's tram trains and sinking of the line are equally delusional.
We will have to wait and see if anything comes of the light rail proposal mentioned (barely!) in the recent budget. The lack of any sort of reliable pre-feasibility style study for it concerns me - reminding me of the various farcical "we're gonna build a metro!" plans announced by the previous government.