Saving the earth is a complex aim with so many different areas to prioritise. Synthetics may produce micro plastics, but then natural fibers like cotton require huge amounts of pesticides and water, viscose or bamboo requires a shit load of chemicals. Carbon footprints for almost all textiles are big. Recycled plastic is a low carbon, no pesticide, low impact material with one downside - micro plastics.
Polyester is one of the few success stories around recycled plastic. It’s not like the alternative is to turn your old plastic bottle into a new plastic bottle, plastic recycling is almost always downcycling the material into a lower grade product.
Like most things around conservation and the environment, it is complicated and there are loads of stakeholders - biodiversity, micro plastics, chemical accumulation, CO2 and greenhouse gases, deforestation, waste management, just to name a few.
One thing we can be sure of is the best thing is to not buying new clothes. In fact this philosophy should be applied to every consumer good. Making things is extremely bad for the environment. Patagonia have been pretty consistent around this point, for example as far back as 2001 they had their “don’t buy this jacket” ad: https://blog.yorksj.ac.uk/jovanalleshi/patagonia-sustainabil...
This is the elephant in the room that most consumer goods companies don’t want to talk about. They can make things out of bamboo or produce paper straws or have recyclable packaging, but fundamentally even environmentally friendly products have a huge environmental impact.
Ultimately Patagonia’s mission statement is at odds with the reason it or any company exists. Saving the earth isn’t profitable.