Then technically you’re calling in: which is the only way where you can use the top speaker separately: when your on a call.
And, do you know how abundantly few people would think this is a “wonderful feature which I ought to use?” Of course, I’m talking about holding a phone to your ear for an extended period of time, listening to a radio station. And, holding your phone up to your ear each time an app plays a sound.
This may be Apple’s POV. It just seems unhandy for the user, what you’re suggesting, but to each his own. You can request the feature at the link above.
Good Day!