In the United States, according to another forum I use, they steadily reduced the bitrate of the SD transmission until it was virtualy unwatchable, and at the same time steadily reduced the bitrate of the HD transmission until it was equal to the old SD standard. By then everybody had forgotten what SD used to be like and was happy to pay the extra for an HD channel to provide them with the same quality as the old SD.
Now the same thing seems to be happening in the UK. Already I understand that Channel 5 has reduced the quality below that required by its licence, and they have been given by Ofcom the option either to improve it or become a subscription-only channel; they are more likely to do the latter. The other channels, in decreasing numerical order, are rapidly following suit - after all, BBC is now led by the man who drasticaly reduced the bitrate of Radio 3 on DAB and claimed, despite thousands of complaints, that listeners would be unable to hear the difference. On that occasion the complainants won and the bitrate was increased again, but R3 has a high percentage of audiophiles in its audience. For other channels, especially the TV channels, the masses probably can't tell the difference and we can look forward to the quality continuiing to deteriorate.