I would agree with you if we were talking about leetcode style data structure/algorithm questions, but system design is almost always relevant. I found 'Designing Data-Intensive Applications' to be a useful book beyond interviewing.

Even if you are not working on a product that requires supporting millions or billions of reads / writes, knowing what is overkill and what isn't, for your project's use case is still useful

Nah, you use similar knowledge all the time in lots of different systems in different fields. You’re not designing a production system in an interview scenario; you are demonstrating critical thinking, showing that you understand how to break down a problem and sketch out a solution, and can clearly and thoroughly explore the different constraints that need to be considered. These are general purpose skills.

Why do you think system design knowledge would not be used on the job? New systems are built all the time. Existing systems are extended/refactored all the time.

System design interviews IME are very applicable/productive actually. Especially ones that put you in a non-greenfield scenario.

Getting good at those requires studying topics that actually are relevant to the day to day job. The Kleppmann book referenced above is one of the bibles IMO. A software book that's actually worth the paper it's printed on.