The problem often is that scrum (or any iterative process) fails when it's not paired with good XP engineering practices. There's constant learning and change on the product side, which drives constant learning and change on the implementation side, the code needs to be constantly refactored to best handle the changes.
TDD/yagni/refactoring allows you to keep your code base nimble. I can't imagine doing an iterative process by slamming features in one after another without refactoring the code as you go to handle what "you now know", or not having good test coverage to support the changes you need to make.
Refactoring also shouldn't be something the PO is aware of, it's not a story, or a 2 month break from feature development. It's part of the job as each story is implemented. Yes, sometimes you don't notice a good way of doing something until some time passes and that area of the code becomes a bigger refactor than usual, just have to deal with it as soon as possible.
TDD/yagni/refactoring allows you to keep your code base nimble. I can't imagine doing an iterative process by slamming features in one after another without refactoring the code as you go to handle what "you now know", or not having good test coverage to support the changes you need to make.
Refactoring also shouldn't be something the PO is aware of, it's not a story, or a 2 month break from feature development. It's part of the job as each story is implemented. Yes, sometimes you don't notice a good way of doing something until some time passes and that area of the code becomes a bigger refactor than usual, just have to deal with it as soon as possible.