What is the main argument by the community not to provide an additional layer of privacy onto Bitcoin? There've been scaling improvements, fee cost improvements, and some small privacy improvements (coming soon). I assume the technology is available to add additional privacy to the protocol through zk proofs or something of the like.
It's more of a "we haven't thought about it" stance given the abundance of mixers and Tor nodes. I don't think anybody seriously contemplated adding an entire layer (think LN, or Liquid) on top of bitcoin for this.
Layers are optional parts of the network and thus only fully work if everyone is using it. If you have a percentage of people not using the layer then the layer won't be fully effective. And for a layer that's supposed to implement privacy you'd need a very high adoption %.