SDTV is not going away. Analog (NTSC) TV is going away.
When we convert to digital (ATSC) TV, we will still have mostly SD, just provided digitally. Some will be HD, but by no means all, or even most.
The cable rule against degrading HD (didn't know it passed) does not apply to satellite. I'm sure satellite will continue pretty much as they do now, with some reduction in HD signal quality, but still way better than SD