Decreasing Volume, Muting, or Pausing... are things the App shouldn't be able to do. It should be the OS that's in control of the state the phone is in, and what actions to follow and allow.

It's always a balance between control in the OS and control in the Apps.

Too many controls/features in the OS, and you get bloat. The device can become less responsive, require more performance, cause more heating, drain more battery, look less intuitive. Think TouchWizz. The best Android OS out there in terms of features and control (well, excluding AllianceROM and PAC-ROM).

Too many controls/features in the Apps and you get "fragmentation". The device can be less secure, it might require many Apps, those Apps may introduce unneeded extras leading to bloat, it might become trivial to download so many Apps to do certain tasks etc etc. Think 4.0.3 AOSP. Is functionally identical to a 4.4.4 Nexus device. However, there's many little features the new Nexus/Stock ROMs include that aren't in the AOSP ROM. And to get that, one must resort to using MODs, Apps, or Xposed Framework.

What's my take on the issue?
I want my Android to be able to handle Health data, Security etc etc in the code. But that doesn't mean I (or the majority) would use it. So its better to keep a lightweight framework inside the code, but offload as much as possible to the App. So that I can pick up a brand new Galaxy S6 and it wouldn't have S Health, but getting it would be easy as 30 second App find&install.

And when I'm in the dark, and I want to quickly activate the device's Flashlight. I want to be able to do that by a basic gesture or by holding down the Home button. This is simple, useful, and should be in the OS. I shouldn't have to waste 30 secs to obtain such a understandable feature.

So its a balance.
Make it possible to make the device powerful/feature-full. But make it convenient to gain these features.
And leave the obvious/useful features baked in the device. AOSP is too light. Android One, Nexus, GPe devices are somewhat light. SONY and Motorola are well balanced. HTC, Lenovo, ZTE, Huawei are somewhat bloated. LG and Samsung have a LONG ways to go.

As bad as it sounds, Apple seems to be striking this balance fairly well, as iOS rather clean yet its very powerful with the use of the intended Apps. Usually the Apps themselves aren't too bloated either. So this balance doesn't just apply to Android, but to iOS as well as any other modern mobile OS. Jolla would be wise to always remember this.