Some still do. Kyle DeVan, Colts starting OG, played in arena league 2 before getting noticed by Indy.
It really wouldn't. First I'm pretty sure the NFL has an age limit anyway and if you think someone like Colt McCoy isn't tough enough to play in the NFL after five years of college I guarantee you anyone making the jump from high school football to the pros, even with some short ass stay in a developmental league, someone like Brian Urlacher would take their head off on the first play. I know every NCAA football team doesn't run pro schemes but it's worked for this long as a way to evaluate players for the draft, why it up now. All the NFL needs is a rookie pay scale and their method of evaluating and signing young players is completely legit.
NFL DL would only be useful for players who didn't play in college or were recovered from an injury and wanted to get back in the game. If you play college ball and can't win a spot on a team's roster, you're probably not good enough.
Your team was awarded a home playoff game with a .500 record two years ago thanks to the division winners guaranteed home game rule (and Denver choking on for 6 games in a row), I really think you're in no position to about this. The way the NFL is now is fine, they don't need to it up by adding more expansion teams or regular season games.