The core problem is that a browser does not know about the APIv2 protocol of the IC. Thus in order to empower a clueless browser to speak to the IC, we let the browser contact the Boundary Node. The Boundary Node can now translate the ordinary HTTP request to a IC protocol conform request or return the IC service worker to the browser. The IC service worker enhances the browser to speak natively to the IC. If we would replicate the state at the Boundary Node level we run again into the problem that the browser is clueless on how to speak to a replicated system – in this case the Boundary Nodes.
By providing a web extension as @faraz.shaikh pointed out, we provide the knowlege on how to speak to the IC via a different path. (That in principle could be manipulated – however installing a web extension is a more explicit step and unlike a service worker that is frequently reloaded, a web extension needs to be updated. Thus, an explicit step, where a security sensitive user could take a closer look and inspect what is updated.)