When does it make sense to invest in content quality? When is the time and expense—the research and interviewing, editorial review, structuring and optimisation, UX and design—worthwhile? And when is it not necessary?
All content has a minimum quality threshold that needs to be surpassed in order to generate any results. The "height" of that threshold is influenced by a host of factors: the maturity of the search results, and the quality of the existing competition; the complexity of the topic; the expertise and expectations of the audience.
All marketers need to aim to clear the minimum quality threshold for every article. Falling short of this threshold represents a complete waste of resources. Take SEO content: if you fail to meet the basic search intent, you can expect to join the 91% of content that generates no visitors.
Beyond that threshold, better quality usually generates better results, but at a diminishing rate. There is a sweet spot where extra time and energy generate meaningfully better results, where the juice really is worth the squeeze. But past a certain point, it takes lots of extra time and effort to eke out even a tiny improvement in performance.