[minimaul]

The argument for this I come up with is rather that they want to maintain the security of their platform - protecting things like their App Store DRM as much as possible.

By disallowing downgrades you can't get back to a version with older exploitable bugs.

[scarface74]

Seeing that you can still download “the last compatible version” of apps with a first generation iPad running iOS 5, I seriously doubt that’s the reason.

Unless you believe they use s separate DRM scheme to download apps on iOS 5 than iOS 12.