I found this pretty interesting - colleges are the vehicle to the pros primarily because they have invested far more resources in becoming that vehicle than any other player in the market
I can envision a day where a player goes to something like a Peyton Manning passing academy and his performance there be just as important as a college game but b/c of that investment you speak of it would take a long time to actually have any significant alternate route to the NFL - huge barrier to entry for anyone trying to capture that market
The problems are really multifold. First, there's the money issue which is huge. Football is the most expensive sport and most Div. I teams run a deficit to field one (only the big guys turn a profit). However, even with big money backing more than one 'new league' complete with TV contracts/regular viewing schedules the product on the field has been subpar because they have to cut corners somewhere when it takes years to build an audience for a team. Fans are tribal by nature.
Then you have the development time, athletic facilities/trainers and stadiums which you either have to build or rent. Men develop far differently from women and often aren't fully grown and filled out until their early to mid 20s and it's even worse for athletes as they have to reach a pinnacle regular men (and women) don't need to. Even NFL athletic facilities look subpar compared to college ones basically because by that time an athlete is already trained (as many athletes will attest the jump from HS to college is much harder than the jump from college to the pros).
After that you have other issues, maturity, guidance, coaching/molding young men into real adults which college coaches are expected to do by the very mission of most uni's. A d-league really doesn't care about that.
Then there's the resources uni's can provide at a cut rate or for free to their schools. Researchers from all kinds of fields giving unis free info. first, free housing/food, nutrition experts and machinery who can evaluate an individual's body and give them a personalized plan, sports medicine as many are attached to their own hospitals, even groundskeepers who are already working in ag departments, and so much more.
And that's just the tip of the iceberg. Young men making peanuts in d-leagues and the minors often have more issues than college kids because they have little guidance and have to pay for so much.
It will be a long time before arena teams etc. offer a better a deal. It can be done but it's a zillion times more difficult in a sport where very few players, even SEC players, actually make the jump. One of the many reasons why these young men ought to prepare for not going pro as well as why the benefits of being a football alum even if you didn't become a true alum (as in graduation) are a better deal in terms of landing jobs. With schools like UT offering a free ride to those who left early and come back to get their degrees, the deal is even better.