Like so much of Ed Deform: It's all about money. Senior teachers are higher on the pay scale and cost districts more money than younger inexperienced teachers. Krashen argues that this is the only rational argument for dumping experience over youth since veteran teachers generally do a better job. They have more years of on the job practice. They have more experience from workshops, professional development, and collaboration with peers.
However, there is one more reason to dump older teachers: Control.
Experienced teachers are less likely to go along with every hare-brained ed deform plan concocted by their administrators. This is one reason why charter schools like KIPP are able to get their teachers to work weekends and summers and be on call well into the night.