A Postdoc in a higher school than where you graduated is definitely par for course, but honestly it’s one more gambling step with at best 30% chance of netting you a good academic career. Your postdoc can still be underwhelming and also unless your PhD was stellar, you’re not betting a postdoc in a top lab anyway. The point is, given that you can’t explicitly choose the lab you want to do a PhD in, it’s just better to try and increase your chances by going to a top institution. If that’s already a hard proposition then the competition only keeps getting worse over time.