The Minor Leagues are there for teams to see who is good enough to handle Major League, more that developement.
There are some very good ball players who just never can adapt to the needs of the Big League but spend many years in AAA.
Some are there as insurance if a player is needed, now, but in AAA they are not getting any development unless they are facing Big League pitchers down there to recover from IL .
Many of he Twins from their World Series team were full time in the Bigs before their 25th birthday, while some Twins from the past were in AAA until 29 years old and finally came up and stayed in the Bigs, not going up-down-up-down-up etc.
Right now Julien and Miranda are there because they suck in the Bigs, and Lewis probably should be there.
It has zip to do with development, and every thing to do with a lack of talent at the Big League level.