there is no evidence that increasing the term of protection for copyright and related rights would incentivise either the creation of new copyright works or the dissemination of older works (which are the primary policy goals of copyright protection).The sole good news is that the extension will not apply to works already in the public domain. Combined with a 15 year implementation period, this means that if we drag our feet, we can get those works out and in public so that everyone can use them. That implementation period is also 15 years we have to lobby for the change to be reversed, and for a shorter copyright term in general.
So why did we sign up for it. Essentially because MFAT negotiators gain status and prestige from cutting deals, and saw this as something they could just trade away. But it also seems to be a perfect example of undemocratic (and hence illegitimate) foreign policy, and an example of why we need transparency in trade negotiations: so our government - or our unelected bureaucrats - can't trade stuff away without our consent.