In my opinion, barring NESA adjusting the external exam weighting which is problematic as you note, the next most fair option is to retrospectively reweigh the term 4 exam to 100%, if no other exams can take place (at least for maths and sciences). This is very different to including year 11 marks, as we are told that
under no circumstances will preliminary marks count i.e someone who chose to study less in year 11 would be substantially disadvantaged at no fault of their own; whilst students are under full knowledge that their term 4 task will count 25-30%, and thus are under the expectation to have studied substantially already.
The reweighing of the term 4 exam would be fair as long as (a) it is an exam and (b) it follows and has achieved necessary exam-making principles, including
- Sufficient testing range i.e the test is long enough (for maths and sciences, optimially more than 60 marks worth of questions) and spreads the entire difficulty range corresponding to students' ability
- Appropriate spread of marks i.e ~50% mean with a wide distribution
- High point biserial for most questions
For the sciences and maths, assignments would not be suitable as replacement tasks because they are generally not able to achieve more than one of the criteria above. For example, one would struggle to create a math assignment that has a wide distribution of marks that accurately affects students' math ability, given the technological resources and group-sharing enabled in recent times. In fact, should such replacement assignments comprise the large proportion of a school's internal mark (e.g 75%), it may compromise the data given by the one exam that has taken place.
I agree with you and do see NESA holding final exams in the place of restrictions, especially by the time we are starting to come out of them. I don't think similar exceptions can be said about school internal exams though.