Hi frosty987,
Thanks for using the Apple Support Communities.
I understand your frustration. Because there are people use Do Not Disturb mode and forget to turn it off, that's why in iOS 12, if you simply "Do Not Disturb" toggle in Control Centre, it will turn off in certain time.
If you want to turn on "Do Not Disturb" mode all the time, simply go to Settings > Do Not Disturb to turn on Do Not Disturb manually.
If you wish to provide a software feedback to Apple, you can share it at https://apple.com/feedback, they may adjust this feature in the future iOS update.
Best Regards,
Juitse
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Resource for this issue
Use Do Not Disturb on your iPhone, iPad, and iPod touch - Apple Support