It's an interesting debate - what is sustainable?
In my opinion, and I accept that there will be many with other opinions, radiata pine has many sustainable attributes. Chief amongst those is that given the right treatment or protection its life in service as a building material can exceed the time that it takes to regrow and be harvested again by 3 or 4 times.
This compares with other timbers that might last much longer without treatment but will probably take 3 or 4 or more times as long to regrow as a minimum.
All organic materials begin to decompose from the moment they are harvested. To increase their life as building materials we need to take a very unnatural step and interrupt the decay part of the cycle and that usually means chemical intervention or a physical barrier or both.
Radiata pine grows very quickly - one of its greatest attributes but this also gives rise to one of its greatest weaknesses, a very open and porous structure which takes in moisture more readily than other species and will decay faster than other species, however this also makes it very easy to treat.
Your comments regarding the hazards of treatment are entirely correct, particularly in relation to CCA treatment. The Jenkins A-lign product that I referred to is LSOP treated which creates fewer issues than CCA, some information on its recommended use http://www.dbh.govt.nz/codewords-20-article-4
The industry appears to be focussed on reducing the toxicity of treatments which can only be good, and the NZBC was amended recently to reduce the use of CCA treatment in timber wall framing in favour of Boron treatment which is much more benign.
If you take the approach of building your own one off super eco home then it is easily possible to scout around and find all sorts of small scale sustainable alternatives to pine. But if you take an industry approach and recognise that we need vast quantities of building materials and that the majority of our plantation forests are pinus radiata then we need to work with what we have got in the best possible fashion.
I regard pinus radiata as a building product that can be supplied and used on a sustainable basis, having said that I would like to see continued improvements in its treatment and reduction in its ecological impact.
If we were to turn our backs on pine then we would be faced with importing more timbers from overseas, or using more petroleum, cement, and metal based products in its place.
It's not perfect by any means, but I do see it as an important part of moving mainstream building techniques towards a more sustainable future.