First of all sorry for the codeless question, but I like to clarify one thing.
I have a senior developer in a team who is actively pushing for code quality - merge request reviews, no crappy code and similar. But most of the other guys in the team has - get shit done mentality. Me as a business guy, I do not check the code at all, but If I would not have that guy who cares about the quality - in my opinion we would hit some heavy refactoring cycles at some point.
But of course there is a downside for caring carefully about the quality too much - it just takes time to do that. Maybe we will have to throw a lot of beautiful code when we will have to pivot as business needs changes.
Two questions: a) how do you keep the quality of your product? What practices do you use? b) Where is the line of caring about code quality enough (not too less and not too much)?
Code quality is important independent from whether you develop agile or not. You are absolutely right that quality improvement requires additional time. Most people fail because they spent their time in larger blocks ('refactoring projects') to more or less clean up code in arbitrary places with the objective of reducing the number of quality issues as much as possible.
The process I advise to use is to follow the boy-scout rule of always leaving the code that is changed a bit cleaner (better) than it was before. That means whenever you change a function, procedure, method or other code unit, fix the quality problems in it. The advantage is that you already understood the code (because you had to change it anyways) and it is going to be tested (because you need to test your original change). That means the additional effort for quality improvement (adding a comment, improving identifiers, removing redundancy, ...) is very low. In addition, you are improving only code that you are working with and don't waste time improving code that is never touched anyway.
Following the boy-scout rule ensures that the quality does not decrease, but instead steadily increases over time. It is also a reasonable level of 'caring'. I wrote some more about this here. In addition you need a good quality analysis tool like Teamscale that can reliably differentiate between legacy problems, new problems and problems in the code you recently changed.