Long has the NFL been without a traditional developmental league, apart from the NCAA.
But MLB and even the NBA have had long term sustainable options for developing players who were not immediately ready for the big leagues.
Minor league baseball and professional basketball in Europe or China have provided, among other things, opportunity for precious repetitions that are not available for those hoping to play pro football.
One of the biggest questions the XFL will answer - whether it wants to or not - is if this has any sort of potential to grow those players who are in the top 1% of the population but not the top 0.