The SD picture on my ClearQAM tuner with my cable company is better than the SD picture over DirecTV. Though the local cable company is putting 10 MPEG2 SD channels in one RF space. 38.8mbps / 10 = 3.88mbps per SD feed (mpeg2). DirecTV obviously doesn't have the bandwidth on 101 where a lot of legacy single lnb dishes are pointing. So they must be cramming a lot more per transponder. Not entirely sure the Mbps on a transponder, but since they are doing MPEG2 it's not efficient as it could be.
Just because they have 10 SD QAM channels in a single RF channel doesn't mean they're all the same bandwidth. They might have shopping channels or news channels that can get by with a lot less, and use more for others. Or use VBR on them all and dynamically change the bandwidth used by each as needed, similar to network QoS schemes.
I don't ever use Directv SD channels or cable SD channels (except a couple like TCM that my cable company doesn't provide in HD) so I can't compare them, but since they publish their mapping for the clear QAM channels I can see how many they pack in per RF channel. I see one with 13 that besides WGN, has a mismash of shopping and religious channels that don't need much bandwidth. There are others with 10, 11, 12, 9 and 5. The one with 5 has ESPN, ESPN2, CSNCH, and NatGEO, along with the Weather Channel. Somehow I'm betting it was no accident they did that, and that the Weather Channel isn't getting the same bandwidth allocation as ESPN
I'm honestly not surprised Directv has reduced SD quality over the years and would expect they'll do it more and more even if they have the bandwidth for it. Great way to get the SD laggards to upgrade to HD receivers. People who still have SD receivers they're watching regularly in 2013 are most likely people who don't care about the quality of their TV experience, don't like to spend much money on their TV experience and/or just don't care all that much about TV in general.
They're the most likely candidates to cut the cord, and least likely candidates to spend extra money on upgraded packages or PPV movies. I'm sure there are exceptions, but in general you'd expect the people who care more about TV and the TV viewing experience and are willing to spend money on it to have HDTVs and HD receivers. Directv probably makes very little from most of the customers still clinging to SD - no extra fees that everyone with HD and DVRs pay. I'll bet they wouldn't be too sorry to see many of them go. Looks like they may be helping push them a bit in several ways:
1) Changes to the protection plan etc. to make it easier for customers still on SD to upgrade to HD or Genie
2) Obsoleting all the receivers older than 2004 or so via disabling the old guide
3) Perhaps slowly degrading SD quality to further incentivize upgrades to HD, and get the customers who refuse to upgrade to voluntarily leave