The NFL is never going to give a provider the exclusive rights that only a subset of their customers (aka only those who have a X1) will even be able to access. It would be like if DirecTV made Sunday Ticket a HS17 exclusive or if Dish got it and made it a Hopper 3 exclusive. Not to mention having the X1 requires that you also get internet from Comcast, so it isn't available to their television only subscribers who rather use fiber via their telco provider since it's faster than Comcast in many areas, or people living in areas where the X1 isn't available at all, since Comcast has a history of neglecting some acquired areas. Plus the X1 "exclusive" channels are usually limited to things that tend to be distributed OTT like Cheddar, or lesser channels who rather take the limited X1 only availability vs not being carried at all on Comcast like I24 News and some international packages, they wouldn't do it for something major like the NFL.
Not to mention, Comcast does NOT have a nationwide footprint, there will be major cities, including NYC, Dallas and most of Los Angeles, where it won't be available at all, along with other NFL cities like Buffalo, Charlotte,
Cincinnati, Cleveland, Green Bay, Indianapolis, Kansas City, New Orleans, Phoenix and Tampa. Even if they do it via iNDemand, who handles the NHL, NBA, MLB and MLS packages for digital cable and telcos, they would be at the mercy of individual systems, since there's tons of systems who still don't offer the out of market packages in HD, or don't offer out of market packages at all, including systems lagging behind on upgrades owned by the 3 major providers who co-own iNDemand Comcast, Cox and Spectrum. Not to mention to consolidate their C-Band transponder space, iNDemand recently switched to 720p only for their HD feeds, even for games that originate on 1080i networks.
The NFL is its own monster that requires dedicated feeds, so for satellite it isn't just remapping RSNs they already carry with some bonus feeds of non-carried channels. For Dish, they would need the bandwidth set aside on Sundays for 15+ additional HD feeds on both arcs and 15+ additional SD feeds for the western arc (since the NFL won't let them make the package HD only). While each cable system would need additional dedicated feeds since many systems do not support IP/switched delivery for their entire subscriber base, if they support it at all, and because of the schedule overlap iNDemand can't just repurpose their existing GAME and TEAM channels for NFL.