IF such a thing happened, and IF the new entity changed the rules or equipment, they would likely accelerate their losses of subscribers. And I think all concerned know that. Which makes such a venture quite unlikely, IMHO.
If Dish came out and said they’d struck a new deal, and you had to watch your DVR’d stuff within 90 days, including stuff already recorded on EHDs- well, even I would likely cut that cord.
That seems unlikely. This hypothetical new company would be solely focused on running a satellite TV business. They wouldn't shoot themselves in the foot by unnecessarily running off subscribers like that.
But what would this new company look like? Lots of unanswered questions. For instance, would it maintain the separate DTV and DISH brands (along with their existing channel packages, pricing and policies) for some amount of time before merging them together? That seems plausible.
Or, OTOH, maybe the DISH brand is immediately killed, with the DTV brand taking over the whole business (since DTV has more subs than DISH, and AT&T would be the majority parent owner of the new company). But DISH subscribers would be grandfathered into their existing channel packages for as long as they want to keep them. Obviously, both satellite fleets would have to remain operational for years to come, allowing all the existing customer equipment in use to stay put.
Regardless of what happens with the branding, channel packages, etc., it would make sense to standardize all
new customer installations going forward on one set of consumer equipment, with dishes pointed at the superior DTV sat fleet (since it has more system capacity and will last longer, thanks to the newly launched T16). The DISH Hopper is arguably regarded as a better DVR than the aging DTV Genie, so maybe we'd see all new installs get a new "DirecTV Hopper," i.e. the existing DISH Hopper with updated firmware made to work with the existing line of DTV dishes, LNBs, etc.
Regardless of whether such a deal happens, I expect that we're going to see AT&T introduce before long a new default set of channel packages (a revamped Plus and Max, plus maybe a Starter package at the low end, with HBO Max automatically included in all packages) that will be sold to new subs across both AT&T TV and DTV. If the proposed deal above went through, those packages should be available to customers on old DISH equipment too. The DISH sat fleet already carries nearly all the channels that happen to be part of the various AT&T/DTV packages anyway, so they should be able to make it work (although maybe customers who want certain add-on Extra Packs, e.g. niche sports channels, would be forced to get their dish and other equipment switched out if the company didn't have room to squeeze in every last DTV channel on the DISH sats).