This is definitely deliberate, upgrading everybody (almost) at once could work, but would have been a huge risk at their scale. You want to roll out big feature

This is definitely deliberate, upgrading everybody (almost) at once could work, but would have been a huge risk at their scale. You want to roll out big features like that incrementally so that you can monitor the load, catch any issues early, and give yourself a chance to revert if things go wrong without pissing off the whole world.