Seriously, IMO Bidkat raised valid arguments here.

Under my sense of justice it's also very questionable to give points against paid services under a certain condition of the worth of them and later change the worth drastically.

And that especially becomes true if you earned the miles on a mileage run. Or if you bought certain products mainly because you could earn miles on them.

For another view think about the business-partners of a mileage programm: They buy miles for a certain amount of money and the airline just decides to raise award levels. That is even more a point where a lawsuit would be in order.