A locked-down device is pretty much definitionally a subscription and not a perpetuity. You cannot fix a locked-down device yourself, so the vendor has to do it

A locked-down device is pretty much definitionally a subscription and not a perpetuity. You cannot fix a locked-down device yourself, so the vendor has to do it for you. When the vendor stops doing that, all of the "benefits" of it being locked down (DRM-protected media, games with anticheat, secure bank access, etc.) cannot continue. I can see the case that locked devices should be easy to unlock when vendor support ends, but then they'll lose most of their functionality too, since the primary reasons for owning them in the first place go away with the unlocking process. An old tablet that can't access any content is little more than a really thin brick.