Code was malleable only to a developer with great code comprehension. In my experience, this was actually very rare. I think part of the reason is, you don't get any recognition for modifying a few lines of someone else's code compared to "I created this new library/app to solve this problem for our org!".
People with poor comprehension have been changing code for years, and in great numbers. Doesn't mean it goes well, mind you.
The big trick always was to limit your scope. If you can't do that or if your tooling doesn't help you with that then very quickly you will run into the limits of what you can still keep in your head.
> Code was malleable only to a developer with great code comprehension
This used to be what being a dev was, if I recall. It's been a few years.