I think the lack of development people are seeing is simply refusing to acknowledge the development that has happened. We've had some great players on both offense and defense during Tomlin's tenure, and he never gets credit for them. Any time we have a great talent, it's always in spite of Tomlin and never because of Tomlin.
I'll just list players CURRENTLY on the team that have developed under Tomlin:
Watt
Heyward
Tuitt
Highsmith (currently ahead of Dupree's development)
Sutton
Alualu (went from a failed first round to a HUGE contributor on our team)
Juju
Johnson
Gentry
the entire OL that continues to get better every game
Claypool (continues to improve and learn)
and this doesn't even take into consideration of players like AB, Dupree, Sanders, etc...