Earlier quoted context omitted.
The biggest problem are apps that do both. For example, I want Uber to notify me when my driver has arrived, but I don't want it to notify me when they have a special 10% discount on my next 5 rides. It's not straightforward to block one but not the other.
The user legitimately considers the application as hostile - hence sandboxing... Notification spam filtering is now the obvious need at the sandbox's edge, with the whole customizable arsenal we have come to expect for our inbound mail. Of course, Google will not cooperate with anything likely to reduce sacro-sanct engagement !
In fact, Uber on Android does use these notification channels. I just have "All Promotions & Recommendation notifications" disabled, and then "Taking a ride" channel enabled.