1. For "invalid", I used it to the clearly ill-defined `site=terminal` which has no indication on what "terminal" it is. 
2. `=university` is basically used as a university campus. But this is not comparable. Using `=terminal` for concourses is similar to using `=university` for Main and Centennial Campus in HKU separately, when they are continuous. That's actually named "Main Estate" officially, due to confusion with the old "Main Campus". 
3. A `=site` + `=terminal` and `=terminal` polygon referring to different things is contradictory. You can notice there is a `perimeter` role defined for `=site` . Main T1 doesn't fit that. 
3,4. If you see a possibility for main T1 and NSC to be 1 `building=` , isn't that conflicting with your preferred definition of `=terminal` to be 1 `=terminal` + `building=` ?  
6.1. As I said, OSM has more precise, technical, and professional treatment. Other apps will have their own limitations, and they don't care about what is `=stopway` etc either. If you search "concourse vs terminal", you can see many results, and how they can be treated as different levels. You can see the same thing in HKIA officially, where they put all the T1 concourses in 1 tab. https://www.hongkongairport.com/en/map/
6.2. A `=terminal` covering all the `=concourse` shows what is accessed from the terminal completely. They can see the different `building=` without the need of a `=terminal` . It's also not exactly the same for navigating passengers. In the same main T1, they have to take the APM to West Hall for NW and SW concourse, while they can walk from East Hall to T1 Satellite Concourse directly. What matters is reaching the airport, where they only need to choose between T1, and T2 in the coming future. For the latter, the 2 buildings are called T2 and T2 Concourse. Making them different `=terminal` doesn't aid understanding. They can only go to T2 terminal directly. 