Reasoned argument coming your way
Well, for a start, unlimited means unlimited, check a dictionary. But as you say, put that aside..
Why are only the top 5% "mickeytaking freetards"? What about those that are only in the top 6%? Are they lovely honest paying people? Why is it OK to make a throttle applicable to a moving target? Put it this way, if July's top 5% all go on holiday for a week (could happen), suddenly Mr Reasoned Downloader in the top 6% moves up into the bad-boy category, without changing his own behaviour. Sound measured or fair? Try arbitrary and unpredictable.
I'm all for charging based on bandwidth consumed. But why not just move to an actual model where you can pay for a proper unlimited package? ISPs already understand there's no such thing as unlimited, even when they claim to truly offer it. Sky is the one example I can think of, that for the moment is able to sell a truly unlimited service. But everyone knows that at some point it will become problematic and they'll reintroduce a similar solution - by which point they potentially have people under contract. They're effectively moving the goalposts once you're playing, and quite often you're trapped on the pitch.