Well, I don't think we can reach any agreement at all if you say that. I understand where you're coming from, though.Yes, security is the main reason and since they're able to say it's for the good of the consumers privacy they're able to get away with it. As has been said, adding new code/features is going to make the OS heavier, full stop.
It's probably not a popular opinion but I would rather they develop code to make the newer devices as good as they can be. I tend to upgrade every year so "planned obsolescence" means nothing to me.