They stated that the new mater lies would not live up to their requirements on longlivity and quality. Abs which is currently used is quite stable and can and is therefore be used for generations.

The new materials would lead to faded and broken Lego bits after some years and degrade the brand perception. Since new stuff would need to be bought then this would be less CO2 efficient.

For other PET products we usually do not have any longlivity requirements. Plastic bottles barely need to survive for a year and therefore the recycling process is still much better than producing new plastic.

In short: It is better to produce 1kg of CO2 once every 30 years than 0.2kg of CO2 every 5 years.

I am sure there is still some typical greenwashing in the calculations, but it makes sense to me that they do not want to reduce their quality standards.