I pretty much agree with the OP's position, but I'm going to try and argue against it, just to see if there is any runway in that. So I'm going to make my statements a bit more pointed -- so feel free to disagree violently - no offence!
Position: Focusing on play is good for short campaigns and one-shots, but long-term will give a significantly worse campaign than if you had focused on other campaign aspects like plot arcs, clocks, world-building, rounded NPCs, campaign-specific rules and GM notes.
Evidence: The Great Pendragon Campaign. This book is probably 75% background, info, GM-only, custom rules and advice. I'd estimate that only a quarter is focused on the play experience. If the book simply was cut down to the adventures and player-facing "what happens this year" sections, this campaign would not be the "grail game" (#sorrynotsorry) that it is.
Evidence: The Lord of the Rings vs Other Similar Series. A book example, but I think helpful. Tolkien is not as good as writer technically as others who write epic fantasy. His characterization is solid, but not exceptional and his descriptions are prosaic. But I massively prefer his work to those of others. My contention is that he did not focus on writing a good book, but instead concentrated on world-building, and so although a standalone short story from him would be very forgettable, the long-form epic version benefits significantly from the non-writing focus and wins against writers who focused on the book they were writing.
Summary/Analogy: The non-play-focused part of roleplaying is like building a solid foundation. If you are just staying for a day, then a tent or a one-story shack is fine and fun, and you should focus on making that space as usable as possible for that day, but if you're going to live in a two-story building for years, building a foundation which you are not ever going to see or "play" in is vital.