For truly long lasting software, I feel all code that make up a system should be treated equally; i.e. not divided into "own code" and "third party dependencies

For truly long lasting software, I feel all code that make up a system should be treated equally; i.e. not divided into "own code" and "third party dependencies". Basically extreme form of dependency vendoring. But it is very expensive way of doing things, and would not work well with modern constantly updating libraries. But I do personally consider software maintenance to be an antipattern. I'd rather have my software be correct and eventually replaceable than constantly changing and "maintainable"