In the last paragraph what you said is often what happens due to bad management too. A good developer can be given a task that they barely have time to get done, and as a result the unit tests, and the documentation, and even the architecture suffers, or gets omitted.

Often in shops where just cranking out new features and/or bug fixing is the goal of management, the software continues to degrade endlessly due to all the things you mentioned, because spending time in those areas isn't something the boss finds justifiable expenditure of developer time. Once all the developers who originally wrote the code have left or been fired then the deterioration in code quality can start to go down rapidly until some kind of "cleanup" effort is undertaken, where ZERO new features are created, but things are just cleaned up.

In projects with millions of lines of spaghetti code sometimes this cleanup is completely impossible, because a total rewrite would be easier.