Players with improvement left develop just fine at the BL level IF you play them. As for when you promote, if a player is better than the guy in front of him, and you're trying to win, NOW is a no-brainer. That said, there are situations where the MiL player is just a little better than the vet making 5.8m. It doesn't make much sense to promote if you're happy with the 5.8m guy.