Note that this is just the "not secure" display on the address bar - it isn't the full blocking page you would see for HTTPS with an invalid certificate.
I think the assumption is that for internal hosts you know the risks and can selectively ignore them but you should be warned about the issue on external hosts out of your control.
letsencrypt now supports wildcards, so that plus appropriate DNS setup should make it possible to use HTTPS on internal resources that you create so users other than you and other admins shouldn't need to worry. If you don't want to use an external resource for certificates then you could install your own CA key on the machines that connect to your network and then sign your own internal certificates though this is more work.
For things you really can't do much about, configuration pages for modems and routers for instance, your base users don't need to care, and you as an admin know the score.
I'm happy to have a little extra hassle as an admin so that the default for man-on-the-street users is as helpful as possible. We still need to properly educate people on exactly what "secure" means though, which is a grand task... "Don't put personal or login details into anything marked as not secure" is a starting point but that is a bit too simplistic as the transport being secure does not necessarily imply that the code at the other end is in any way trusted.