i think manufacturing would remain centralised, since the infrastructure is already established & shouldn’t be wasted. demand would shrink due to reuse and recycling so supply could be slowed down massively, meaning distribution could happen slower and in more sustainable ways. assembly however could easily be done locally if we can establish strong universal standards. i think 3D printing (with recycled plastic, at least, if not something plant derived) & FOSS designs would help facilitate this.
they aren’t truly BIFL, i don’t think they’re particularly solid, but AIAIAI make modular over ears - so at least not the full unit needs to be replaced as they age. i use a pair and my only complaint is the bleeding - eg on a train, anyone in the adjacent seats is definitely hearing what you’re listening to. could be an issue with just the cups i chose though