The interest at a major college program comes from alumni and regional interest/pride, that has been built up over generations. While a new d-league might eventually generate some regional civic pride, that's going to take a long time to develop (if ever). The college football rivalries have been built up over decades, passed along to children from the day they were born. Even if the best college players go to a d-league, I doubt very many fans are not going to switch their primary interest from their SEC team to an Orlando or Birmingham minor league team.
How much interest is there in other professional minor leagues? Going to watch the PawSox or Providence Bruins may be a fun diversion, but it is not in the same conversation as the amount of interest to see Alabama or Florida or Notre Dame play football, and it never will.
In New England it might work because college football isn't that big, probably due in part to there being so many colleges. However, in most of the rest of the country fandom for the local/regional college football team is intense; I just can't fathom those people suddenly becoming more interested in a minor league team - even if they had the best college-age players - than their favorite college team.
Between an NFL team, a college team, and perhaps the local high school team, I don't see there being room in an average football fan's social life and free time to also support a minor league team whose schedule overlaps that of those three other teams.