I feel like - opinion, not based in fact, I've never worked on any codebase >1M LOC - the only way you can keep a codebase like that maintainable is by very strict code style and architecture rules - e.g. no functional programming, no feature X, no feature Y, strict code style rules, strict architecture rules, etc.
Personal experience. We have some java code from 15-17 yrs back. About the time “design patterns” and all the other “pattern” stuff was big in everyones mind. It’s a mess of abstraction for simple tasks. Couple all that with the fact that class instantiating is possible via reflection and at this point people maintaining it have kinda given up. It works - don’t touch it.
This isn’t an indictment of Java. Just resume driven design.
Programmers are highly trained like to see the world as if everything can be cast the same way. While that IS a good way to see a large variety of software problems, when you isolate down to a companies' use cases, it quickly goes away in the pragmatic sense. It then becomes something the company is paying for because they chose Java and Java developers like that sort of thing.
Long ago I worked primarily in perl and ... yeesh, I recognize maybe 30% of it when looking, and that's the directly-c-adjacent stuff.
To be fair if I walked away from Rust for a year I'd probably be similarly lost.
The Amazon retail website (amazon.com and related versions for various countries) used the Perl Mason templating system (https://masonbook.houseabsolute.com/) to allow various teams to build web site components in a performant and secure way.
It worked quite well since it ran quickly and isolated components so that only one small change could be made without breaking other components.