[keeperofdakeys]

But then you remember that Ed was designed to be used on what essentially was a typewriter, because dialup was so slow, that you couldn't send an entire screen worth of text at once. So you would basically be remembering the text in your head, while inserting and regexing lines.

And the great error handling when it doesn't know what you just told it to do. ?

[LukeShu]

It's not that they had dialup, or that they couldn't send an entire screen at once (though I'm sure these factors later helped keep ed around). The devices they were using were much more like a typewriter than you seem to think, they literally printed the text out on paper! The programmer didn't need to remember the text in their head, they had the paper right there to look at!