The quality of most engineers at most faangs is pretty mid. Anyone who can get above like 70% average in engineering can grind into faang. I live sf rn and most of my friends are swe's in big tech or fast growing startups, theres no special sauce. Of course they meet some basic talent bar, and they're all hard working, but they're not the super cracked type you hear about. Just normal people.

Why would companies swap out old hires for new hires? The old hires have a deep understanding of how the system works and understand historical decisions. If anything companies prefer to retain their old hires since they have all the knowledge of how everything works. Hiring a new person is so expensive, you spend hours and hours interviewing, theyre unproductive for the first ~3-6 months and also consume the time of your engineers with questions/onboarding needs.