The key here is CHURN!
Dish network generally churns at the rate of 1.5% per month, which is 18% per year, or over a 5.5 year period you are basically replacing the entire customer base.
So what has happened is that enough customers have either already upgraded, or enough customers have churned with older equipment to make it worth while to upgrade the ones who will keep on paying every month and never upgrade.
As far as using MPEG 2 equipment, its Free since its all been used previously on a leased account. Even if they got to go back and change it out again in 5 years, its better to use what they got and worry about the future later.
Keep in mind most of customers who get an equipment upgrade for something like this, are offered a DIU with a new 2 year contract. When a customer declines the committment, why reward them anyways by doing it with no committment? at that point all Dish is obligated to do is upgrade them enough only to get by.