Because if it doesn't, Russia's current border is already indefensible and Russian demographics are already so bad that if Russia were to do nothing at all, it would doom itself as a nation.
Russia is a nuclear armed nation. It doesn't need to defend itself against horde attacks as though this was the 1300s. And attacking Ukraine does nothing to allay it's impending demographic collapse - indeed it exacerbates it. So if Russia does invade Ukraine, it won't be due to any of the reasons you explained.
Good writeup but I disagree with this:
Russia is a nuclear armed nation. It doesn't need to defend itself against horde attacks as though this was the 1300s. And attacking Ukraine does nothing to allay it's impending demographic collapse - indeed it exacerbates it. So if Russia does invade Ukraine, it won't be due to any of the reasons you explained.