Alright, just to drive this point home.
This is what the code for Books 1&2 looks like in the last Azula update from 2017, and how it's changed since then. Filenames that are colored red are the ones that had their content modified, black are the ones where everything is identical line by line.
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You may notice that there aren't any, because both versions are, in fact, the same fucking thing. The code for Book 1 hasn't been touched or "built on" since Book 1 was finished. Same with Book 2, same with Book 3. And I mean to a ridiculous extent, the current game flow is basically "run the standard example project script.rpy which contains the intro and ALL OF BOOK 1, but there's a map screen injected into the middle of it which lets you jump to the other 3 books instead of finishing it".
And this is what FET looks like if you take all of the code and assets for the first three books, and just fucking YEET THEM OUT OF the game entirely:
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You may notice that it looks like the same fucking thing, because none of the books reference any of the previous code and Ren'Py doesn't give the tiniest of shits if something isn't there.
BUT WAIT
THERE'S MORE
This is what the Python code for Book 4 looks like (the part that isn't just linear dialogue or image names), the portion that takes MONTHS to familiarize yourself with, the hurdle any new coder on the team would have to get over, the leviathan task no man can hope to overcome, THE ABSOLUTE MOUNTAIN OF
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it's two files
No seriously, it's two files. That's it. Everything else is either file paths, the dialogue, or basic VN stuff. Here's what the map system looks like:
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Impressed? Check out the day/night system, then.
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ARE YOU FEELING THE WEIGHT YET
TO BEAR SUCH AN UNIMAGINABLE BURDEN
FOR ONE MAN TO UNDERSTAND THIS IN A MERE FIVE MON... you get the idea.