I think for the case of the home router admin page (or really any admin page on your local network), the browser can easily detect that it's being served a page on the local net, and could provide a less scary warning that has some text that acknowledges that you're on your local net and that either ignoring the password form security warning (for http) or accepting the self-signed cert (for https) is probably ok. Whether they'll do this is another question...

It absolutely makes sense not to display warnings that HTTPS isn't in use in circumstances where HTTPS is not feasible - i.e. local addresses. It's simple enough for a browser to determine. I don't think conditionally accepting a self-signed cert is necessary.