Well, I think it has to do with the evidence.
1. The Legends "blurb"
2. An evident "Gag-Order" for 2K employees to not speak of Legends or any football for that matter. (why not just say "no, we aren't making one.")
3. Demand.
Let me focus on #3. There IS demand. Heck, if Blitz: The League could sell a million copies despite the cheesiness and the disregard for anything resembling real football, what could a real football product do in terms of business? Answer- more than a million copies, that's for sure.
Plus, there's demand in other ways too. Online multiplayer franchises are 3 years old since first seen in NFL 2K4.. yet EA ignores them and pretends they don't exist. There's TONS of people that WANT them to exist, yet EA does its usual bit of "nothing" and "more of the same".
(Don't even bring up "The Locker"- those franchises are a "who used the file last/chaind-of-possession nightmare for more than 2 or three players.... and that's when the stats successfully merge from the spawns...)
Demand for 2K football (of any kind) will grow as long as EA keeps releasing products that don't do anything to help fans of 2K5 forget about its features.
EA hasn't "wowed" anybody since Madden 2001.
So 2K has to see the demand. They'd be blind not to.