Speaking as an electronic engineer with an interest in radio and to.
1) NBN and tv cabling are two separate things. For a limited time, NBN will cable your building and each unit for free. However there may be some cases where the OC gets the building cabled for NBN by their own contractor.
2) If your building to cable is the old flat cable, then yes you need to replace it as modern TV sets will not receive signals on it effectively.
Modern TV sets need good quality coax cabling. It has less losses which means less drop outs in the signal on the tv.
A good installer will measure the tv signal at your building and then design splitters and amplifiers so that sufficient signal gets to each unit. If the installer can’t do the design then they’re not for you.
In a 70s block the biggest cost will be the running of the cabling, not the amplifiers and splitters.
As to future proofing. Yeah do the tradies can predict the future? Get what works today and no better.
Good luck