> This is a bit of a tangent but it seems to be a common trend now among labs to arrest trainees (particularly phd students and post-docs) in their positions as long as possible

I've witnessed this as well. One of the worst offenses I saw was a master's student in a 1-year master's program (basically supposed to be a post-bac / used for switching fields) who spent around 4 years in the same lab instead.