Then you are not appreciating boldness of this move.
Many Countries ,have derecognized legal tenders before, but no one has fixed such a sharp time limit on their derecognition. These notes have gone out of circulation in just 4 hours and would become completely worthless (even at bank) on 31st December (or is it 8th as you are typing).
This means that hoarders do not have opportunity of using time tested route of converting black money into white using small transactions over a period of time. Since there is a time limit upto which banks would accept these notes ,people could only convert very small proportion of their black money into white using convention laundering methods, and gold dealers also would not accept these notes as they too need to get rid of them as fast as possible.
It is not derecognition that is bold, but narrow time window for exchange that is bold. There is no way in hell that people would rooms filled of black money in high denomination notes could launder all of their money in such a short period of time without getting caught.