Not all apps have in-app volume. In which case you have Hardware volume x Windows master volume x Windows app volume -- and which should be removed? No hardware volume? Everything controlled by software? Sounds horrible. No Windows master volume? What if you want everything quieter, turn it all down one-by-one? No Windows volume mixer? I explained why that's an issue in my previous comment.
Your example of a good implementation is iOS which is hard to compare to because mobile OSes are generally single-task focused. But I certainly wouldn't want an iOS-style volume control on my PC.
Your example of a good implementation is iOS which is hard to compare to because mobile OSes are generally single-task focused. But I certainly wouldn't want an iOS-style volume control on my PC.