The NFL always needs to have a plan for a developmental league in case the NCAA goes south with all of this payment and breaking off. The power conferences already threatened to do their own thing if the NCAA didn't do as they demanded, the NCAA has recently lost lawsuits, and really, the NCAA doesn't have much power anymore. They tried governing teams and took big PR hits from the fans for that.
Personally, I'd like to see a developmental league exist. Arena football is really exciting, but the rules are so far off from the NFL. The CFL's rules also make it tough for their players to transition. The NFL needs to think up a model that is cheap and grooms talent. The NFL has really failed to market these leagues. The league also needs to be in the USA, because it's hard enough to get Europe to watch NFL games, so why would they watch sub-par talent play? There is a real chance here for the NFL to do something cool, like take the preseason tickets off of the NFL season ticket game list and add them to a cheaper developmental league's season ticket list. That way people can get 2 NFL games (even if they are preseason) and a bunch of developmental games. And heck, maybe give them first crack at a playoff game, too. These are the types of things the NFL can do to make a developmental league successful.
A couple of thoughts:
What the NCAA does or doesn't do is not the issue. The issue is whether Alabama, Auburn, and a handful of other like- minded institutions want to play football on the current scale, whether in the NCAA or outside of it. Whether they have to pay players or not, I believe the good people of the state of Alabama would just as soon see these fine institutions of higher learning fall in a sink hole than give up football on a grand scale. And the money, baby, the money...SEC football TV revenue is huge and getting huger. The NFL's key "AAA ballclubs" are not going anywhere. The colleges will pay the players if push comes to shove.
The NFL might actually like this arrangement...all the best talent distilled down to maybe 30 mega-college programs that can afford to pay the players on those 60 hour football work weeks. Other schools can go back to having student athletes on limited work schedules and perhaps get away with not paying them; the players might even take some real classes and do some studying.
The original developmental league played in the spring with most of the franchises in the US and I think three in Europe. In the US, they were mostly in smaller markets like Sacramento, but also a northwest NJ franchise. They killed the US clubs after a few years and added more in Europe because the European teams were outdrawing.
It was originally a spring league, and spring football leagues have always done poorly in the US. And you can't run it in the fall/winter in the US because Fri. - Mon. is chock-o-block with high school, college and the NFL. Nobody will go to football games midweek after getting over-saturated all weekend. That's why it became an all-Europe league, along with the original hope American football would catch fire there and pave the way for expansion overseas. Nothing worked in nearly 2 decades of trying.
Your idea of packaging NFL preseason games with a developmental league package seems to assume the developmental team will be in the same market as an NFL team. I go back to the competition and saturation problems.