> Our developer brains scream at us to ensure that we’re not copying the exact code three times, we’re creating the header once then “including” it on the three (or a thousand) other pages.

Interesting, my brain is not this way: I want to send a minimum number of files per link requested. I don't care if I include the same text because the web is generally slow and it's generally caused by a zillion files sent and a ton of JS.