That's the problem, it is too late in the life of satellite to try to migrate customers from Dish to Directv, let alone the other way around (Directv has nearly 20 million customers, including hundreds of thousands of commercial customers with fairly complex installs) Given that after T16's launch (in a few hours from now) Directv's satellites wouldn't need to be touched for at least a decade, which Dish can't say, and Directv has far more bandwidth including lots of room for 4K if/when that ever happens, that I can't see it making sense to standardize on Dish's hardware/satellites even if Directv didn't have double the customers Dish does.
Charlie Ergen is a businessman, and obviously a pretty successful one. He isn't going to want to spend an extra few billion retiring Directv's infrastructure for Dish's more limited infrastructure just to say "I won", because you don't get to be a billionaire by throwing away huge piles of money playing childish games.
People keep focusing on "well eventually there won't be enough customers to support two satellite companies". That's true, but you don't have to merge them. One of them will eventually go out of business - profits will be declining, the board will see the writing on the wall and set a date to close it down and sell the assets before it starts losing money. Based on having half as many customers and a more expensive infrastructure (two arcs) that will need replacements much sooner than Directv's, that's Dish. Then its customers who need/want satellite will come to Directv without Directv having to pay a cent. And if it works the other way around the same would apply.
I know there are people out there probably dreaming "if only I could get Hopper on Directv" but the HS17 already has 16 tuners just like the Hopper 3 does. Directv allocates them differently because of the way they plan to do 4K. They are prepared in case 4k really takes off. The longer we go with zero real 4K channels the less likely that looks, so maybe it will be a fool's errand like their preparation for 3D, but a future that has dozens of 4K channels is one Dish is simply unable to participate in due to their lack of satellite bandwidth.
I can't say for sure, but it is possible a software update could give the HS17 15 tuners, if Directv decided some time in the future that there will never be very many 4K channels, and devoting one full transponder per 4K channel would be fine. If it wasn't for their plan to do transponder bonding to allow for even more than the 36 4K channels that one per reverse band transponder would allow, the HS17 would have had 15 tuners instead of 7.