Students leaving a new lab - have I made a wrong choice?
The main result of  a PhD program is not a PhD thesis, it is a PhD-level scientist. In other words, you.

Your PhD is about you learning to be a researcher. It's about things you pick up from supervisor and the lab: research methods, writing styles, professionalism, passion, attitude. 

The postdocs in the same lab may have different goals than you, different experience, different needs and expectations from/towards PI. Their decisions are their own. 

For the sake of example, let me exaggerate a bit. Imagine you also had a  college student in your lab, and your PI happened to be a great college teacher, spending a lot of time explaining some basic maths to the student. The student would probably genuinely  benefit from this, but as a PhD student, you could feel yourself a little bit overlooked.  Is not it what your postdoc colleagues might've feel at the moment? Is it possible for a PI to be equally perfect for all members of the lab? To what extent this all-round excellence matters for you?

Maybe, reflecting on these questions helps you to find the right decision. Good luck.