Q: Are all MPEG-2 essential patents included? A: No assurance is or can be made that the License includes every essential patent. The purpose of the License is to offer a convenient licensing alternative to everyone on the same terms and to include as much essential intellectual property as possible for their convenience. Participation in the License is voluntary on the part of essential patent holders, however.
So each implementer is still on the hook if a new patent holder pops up.