PET is just not a good material for brick toys. They deform quite easily and that goes against LEGO brand. I wonder why they didn’t use HDPE which Green Toys utilizes. Perhaps not enough recycled materials for LEGO scale.
The basic brick uses ABS.
If long-term durability were a concern, ASA might be a better choice but it costs 50% more.
Changing it to be something else would just cheapen LEGO to something that wouldn't permit intergenerational inheritance.