No, Apple’s Health is used ubiquitously by other companies. My Withings scale integrates into Health, Myfitnesspal (which my wife uses) does, too. Wahoo and Strava upload data to Apple’s Health, too, as does Strong, a workout app. I reckon Garmin does, too. You can grant read and write access separately and for separate data fields if you want to preserve your privacy. It really works well. The nice thing is that it acts as a central repository with information that is not obtainable from each of the apps separately. E. g. my Withings scale would not have to integrate into TR, Wahoo, Strava, etc. separately, just once.
DJay does integrate with cloud services like e. g. Spotify and it does have a ML/AI component. As an app, it is much more complicated, because it relies on a lot of time-sensitive operations that need to be optimized for each platform.
You are right that TR needs to develop a sophisticated backend, too, and I mentioned that already. There is a tendency I noticed among many companies (think Netflix) that think their competitive advantage is what is on their servers, and that is true. Another, even worse example are business apps, where usually nobody cares about the end user. But user satisfaction is still in large part determined by the face of TR, which are the apps.
How many e. g. video apps do you love? Do you love the Youtube app? Or the Netflix app? I use both a lot, but I hate both even though I like the services.
You have a point but IMHO make a big mistake: good design is not how it looks, but how it works. I can list a ton of issues that have nothing to do with how TR does something, but basic functionality that is simply missing on the mobile apps. I cannot do a lot of the calendaring. Yes, I can schedule and delete workouts. But I don’t think you can drag and drop workouts around or long-press to get to a menu. I can no longer check whether I hit my power numbers or add annotations.
And I think the most important limitation is the lack of proper performance analytics. I. e. how do you display changes in performance in a way that is relevant and understandable to TR’s athletes? A lot of TR’s future problems are related to how it presents the data obtained from AT to its users.