As someone who doesn’t root my iPhone and would prefer nobody else do so, there is a key advantage to me to Apple’s preventing downgrades of the iOS version on a device. If an old release contains a security flaw that would allow access to secured data on the device which a newly patched version would prevent, allowing an install of the older version would allow an attacker to downgrade, exploit the bug, and extract data from my device. Jailbreaking a device amounts to removing all effective protections in terms of access security that the device may provide for data stored on it. Many (but certainly not all) jailbreaks exploited vulnerabilities at an OS-software level, and patching those vulnerabilities made it impossible to jailbreak an updated device. That doesn’t apply to the devices that had exploitable vulnerabilities in the ROM boot block of course; and that was an issue on several of the devices.
Ensuring that older vulnerable versions cannot be installed on my device is a security feature to me. It’s also a limitation of my freedom, and it makes the device less “mine.” Looking at the alternatives and what I *personally* want to do with (and want done to) my iDevice, I’ve decided this is a better option for me. I’m not prevented from using the device in the manner for which it was marketed, and potentially some would-be attackers are thwarted from extracting data from my device. Certainly there’s more that I could potentially do with the device in terms of home-rolled firmware, but I’m at the point where I really and truly don’t want to “hack” on my cell phone with all the potential issues that come with that (battery drain, instability, insecurity, etc.). I want to pull my iPhone out of my pocket and make a phone call. Apple’s update policy doesn’t prevent that, and their QA is reasonably good in terms of the battery drain and related issues being a relative minority of their users. I’m willing to take the risk of updating.
In a perfect world, boot loader security might be accomplished with a key that’s under my control rather than under Apple’s. Maybe a card / USB stick in the box with the private key for the boot loader and a “lose this at your peril” warning. The device could take official Apple updates without the key (better if that was a configurable option), or take any software with the key. Of course the support nightmare that would ensue would be insane, and Apple would never do it as it’s a definite UX detraction for any but the most geeky of buyers.
RMS’ great vision of full control of all of our hardware is a nice one, but in the mean time I’m content to enter compromises with companies that have reasonable histories of “not being evil” in the areas which directly effect me. I’m good with Apple and how they handle their iThing’s. Sure that could change, and I keep my data off-device in formats that would allow me to convert and go elsewhere if need be. Beyond that, I’m not going to lose sleep over the fact that I’m stuck with one-way upgrades of iOS. It’s one of those arguments that I can agree with ideologically, but in terms of practicality and getting things done, it doesn’t negatively effect me.