I'll give you my hunch, so take that for what it's worth — If you complete the Super Friendly Charisma Challenge and it says friendships never decay, then that means when you'd get a certain relationship to a particular level — it will always remain there — at the highest point you achieved. That's really powerful. The reward doesn't sound quite as powerful. Because there it sounds like a friendship might still decay a little, but just never to the point of someone becoming a distant friend. Again, this is all a guess.