There's also significant overhead to the community at large in having both HTTP and HTTPS be reasonable systems to use, and requiring that HTTP not show loud warnings. There's also a risk to your organization that you're teaching users that some HTTP sites are reasonable, which is a hard judgment for them to make. I can put up an external website which claims to be internal, and probably get some passwords or confidential information that way.

If you use HTTPS everywhere, there is a tiny bit (but usually negligible) runtime overhead, a bit of process overhead (which this announcement is pushing much closer to zero), and significant simplicity in many other axes. I think the tradeoff leans towards publishing internal sites with globally-valid HTTPS certificates.