You sir, have obviously never encountered the problem. The message does one of two things: 1) Gets marked as delivered but is never delivered because the person has no iDevice or 2) Gets marked as undeliverable and is not resent as a text. I have a friend who has been trying to fix this for months and at first her messages disappeared into the abyss. Now they just fail to deliver and I have to manually resend it.
I have encountered the problem, and solved it for friends/family.
Since I'm an iOS developer, anybody who has any problem with their iPhone asks me how to fix it. And since I'm a tin-foil-hat-toting privacy advocate, I have studied various articles that reverse engineer how iMessage works. I know exactly what "delivered" means —it means some device somewhere decrypted the message. Apple's server cannot decrypt the message as they do not have the private key, so therefore they cannot possibly send a delivery confirmation.
Go ahead and try it out. Disable wifi and cellular data on an iPhone but leave the non-data cellular connection active, then send an iMessage to it.
The message will not change to "delivered" unless some other device is registered (and connected to the internet) to receive messages at that phone number. After some minutes, the blue message box on the sending device will change colour to green, signifying an SMS has been sent. Depending how good your cell carrier is, the SMS will be delivered instantly or after a few days (SMS is not a reliable messaging protocol...). This assumes you have not disabled SMS fallback on the sending device, which is the default.
I just did the test, and it proved my theory. Disconnecting my phone/ipad/mac caused a sending device to fail to show "delivered", and several minutes later my phone received an SMS message.
The system is overly complicated, mostly as a byproduct of Apple's end-to-end encryption system, which leads to a lot of customer confusion and miss-information when they try to diagnose one of the many things that can go wrong. But I know what I'm talking about, delivered means it was delivered to a device registered receive iMessages at that phone number.