Maybe but your evidence is annecdotal. Good engineers leave huge companies all the time but it is usually balanced by inflow. I guess we have no evidence of inflow as compared to outflow of good engineers.
I didn't reach a conclusion, I presented a possible reason knowing very well that I don't have the evidence to back it. Hence why I said "could it be that the engineering force at Amazon is no longer what it used to be?" and not "The engineering force at Amazon is no longer what it used to be."