Even in the past, you could use agile. e.g. roads.

It's only constructions like cathedrals that you can't really use until it's nearly complete.

Imagine having to wait until 2015 for Google to ship its first suite of products. The cashflow and other things wouldn't make sense. Google is essentially under constant construction though, as is a University which adds buildings every so often.

This comparison has been made before - "The Cathedral and the Bazaar." It's an excellent book, and I highly recommend it.

https://www.kobo.com/us/en/ebook/the-cathedral-the-bazaar

I don't think that's accurate. There was a whole lot of:

"Ok, remove the support scaffolding"

"Oh crap it's falling! Put it back, put it back!"

followed up by a design tweak.

Or the King/Bishop decided that they wanted something different and the design was changed.