It may seem like Coder could replace ArgoCD, but I think in the long run, you'll see that Coder, or rather Terraform, doesn't really fit that role completely well. It depends on the extent of your system.
You might be thinking Coder actually fits the "CI" role and I believe that is absolutely what it does. In k8s, Coder allows the dev to be able to code directly within the true environment being used, albeit, it should be a staging or dev cluster/ namespace and not the production cluster, as that is what ArgoCD is needed for.
In other words, you'd have something like this,
https://cdn.discordapp.com/attachments/1101265517081264228/1101409660394799154/image.png▾