It's possible that you and I have different experiences. But many times I've seen people at FAANG gunning for that coveted E6/Staff promo choose new technology

It's possible that you and I have different experiences. But many times I've seen people at FAANG gunning for that coveted E6/Staff promo choose new technology even when unnecessary, because it's hard to find projects that demonstrate the skills needed for that promo. The context also matters. If you're working on a small team with low attrition, the cost of choosing new technology is low. If you're on a large team or in a high attrition environment, the costs for choosing a new, unfamiliar technology are astronomical.