[General Questions and Answers] Droidriven:
They lock the devices down for 2 reasons. They are not concerned with losing users like us in this community because we only make up a small part of their userbase, there are not as many of us as you think.

1) they do not want the user or any third parties modifying the device. They are a financially motivated entity, a business, it costs them money to replace devices that the user has tried to modify and bricked in the process, which happens more than you'd think.

2) to keep the device secure, a downgraded device is not as secure as one that isn't. It is intended to keep your device as secure as they can make it, whether you as a user want it that secure or not. When it comes to keeping the device secure, it's more for their concern and security than it is for your security as the user. They are protecting their proprietary product and their proprietary software.

The reason why they can do this is because we "technically" don't own our devices, we own the privilege to "use" the device, the manufacturer owns the hardware and software. It's somewhat explained in the End User Licensing Agreement(EULA) that you agreed to when first signing in to the device and your Terms of Service agreed to when you started service with your mobile carrier.

As for the "bit" that you found, many have found that and thought something could be manipulated there, they were all wrong.

Trust me, it's a brick wall that you'll never get past, all you will achieve in trying to crack it is a bricked device that you'll have to replace. Find something more worthwhile to tinker with. This subject is a dead horse that has been kicked to the point that it is thoroughly dead and will continue to be as dead as a doornail for the foreseeable future.

As for what you mentioned about a user updating stock on their device and not liking it but having no way to return to what they had before. That can be achieved by downloading the firmware from before the update then extracting the update and extracting the original firmware then extracting the updated bootloader from the updated firmware and placing the updated bootloader into the older firmware, then repacking the modified firmware and flashing that firmware via Odin. This allows you to downgrade the system(OS) to what you had before the update without downgrading the bootloader, the modified firmware will have the updated bootloader so it won't block the attempt to flash the firmware. For example, it lets you go back to lollipop while keeping the marshmallow or nougat bootloader, whereas flashing lollipop without swapping the bootloader would be blocked. In some cases, it is a little more involved than just swapping bootloader, other parts of the different firmware may need to be swapped also, but the concept is the same and can be done if you know what needs to be swapped. In some cases, you can just swap the system.img from the older firmware into the updated firmware then flash the updated firmware containing the older system.

The point is, you can't get rid of the updated bootloader, but you can go back to marshmallow or lollipop or even Kit Kat as long as that android version is available for your device. It just requires modification to the firmware file to make it have what the hardware/software actually requires/needs and the OS that you actually want.