I disagree with that statement. I find that a junior dev is far less jaded by the work world and will often develop a loyalty and dedicate to the team (if it is a good environment) that a sr. Dev wouldn't. Once your are 5 to 10 years into your career how you view your work/life balance and professional relationships changes dramatically.
A junior dev will often stay with an organization much much longer than what is probably good for them, especially if they are actively mentored and engaged.
I agree with this. My first job I was under a fairly good partner who took the time to mentor me, get me involved in decision making processes, and take me out to see clients. It was some of the best times of my life and I did learn a ton. I had to move for a family issue and immediately received a 35% raise in a similar COL location. If I didn't have that issue come up, I probably would have stayed much longer and continued to be underpaid.
A junior dev will often stay with an organization much much longer than what is probably good for them, especially if they are actively mentored and engaged.