You nailed it with your last sentence. And those people were not likely to start using Day One and pay money for the privilege. I’m one of those people.
This is like so many other Apple apps, Notes and Reminders in particular. They are ok (and have gotten better), but you’re never going to move a dedicated Omnifocus or Obsidian user to Reminders or Notes. They have already moved past that level.
Will this cost Day One some future users? Perhaps. But it may also spur them to add additional features, making their product better in the process. And people who start out on Journal, may actually move up to Date One once they hit the “feature ceiling” in Journal.
I feel that Day One has gotten complacent in recent years in terms of functionality of the macOS app in particular. Their own sync mechanism is also starting to show some flaws. I've seen sync issues with Day One when starting a journal entry on one device and then switching to another device. Sometimes, the entry is incomplete or missing on the other device. That generally indicates a sync issue, but it isn't always automatically resolved quickly. A comparative (but different) app, NotePlan, which uses iCloudKit for synchronization generally seems more reliable, in my experience.
I wish the best to Day One, but since they were acquired by Automattic, it really seems like they are in maintenance mode and have not substantially done much with the app for years. For instance, the calendar/timeline sidebar
still cannot be hidden completely for a true focus mode in macOS, even after so many years of requests. The journal entry text area in the Day One edit/view window also is inflexible above a certain width, and the developers have not added the option to allow users to set a text area width, after years of repeated requests.
I'm glad that Day One has provided a stable platform for journaling for these many years, and I'm thankful to them for that. I do pay for the Premium plan, even though I really don't use or need all its features. I think that Apple is giving us a new view with their app (currently iPhone only?) as to what can be done in terms of deep integration and prompting.