It would e.g. make no sense at all to disable “Android System”: nothing would work anymore on your device. If the app-in-question offers an activated “disable” button and press it, you might have noticed a warning popping up: If you disable a built-in app, other apps may misbehave. Your data will also be deleted.
When you call a dead phone does it ring?
Answer: A: With a dead battery it should not ring but it should go to voicemail directly.