1. In Silicon Valley, people are not bounded by non-compete clauses and can come and go at will. So fungibility is a top priority for any tech company. The only way to do that is to make sure expertise is shared across the team and not monopolized by one or a few old-timers.

2. Eng teams that have mostly old-timers tend to get stale and slow in changes. This is bad for products that need rapid evolution or new ideas to break status quo. New engineers have way more incentives to make changes to prove themselves and collect credits, while old-timers tend to play safe and stay on the side of stability.

3. Bad coders, not new coders, write bad code.