I think Doug really has hit the key point - the majority of customers are still SD, and DirecTV is still doing SD installs with MPEG2 only equipment. I don't know the actual numbers, but let's just make some approximations for sake of discussion...
Total DirecTV customers: 20 million
Number of SD only: 13 million
That is a HUGE amount of hardware in service that is working just fine, and will continue to work just fine for at least another decade. The cost to replace all of the IRD's for over half of their customers is huge.
Let's make some wild guesses. If it would cost DirecTV $200 to replace one SD customer with HD (dish, IRD and service call to install, one receiver per customer), and using the above guess of 13 million SD customers, then we are looking at $2.6 Billion (with a B) to make that conversion. $200 is probably a very conservative estimate for a customer upgrade.
So, with natural attrition of SD customers (either moving to HD or leaving DirecTV altogether), let's say we cut that 13 million in half in say 5 years. Well, you're still looking at over a billion dollars to convert the remainder.
I don't expect to see the elimination of SD service for a very very long time.
But, that's just my opinion, and it's based on numbers I made up out of nowhere with nothing to substantiate them. Just wild guesses on my part. But whatever the real numbers are, the point is that the cost to convert is huge, and will remain so for a long time to come.