Well, small, lean, and focused also implies that the time to deliver is still a bit longer. If you're willing to wait 5 years for something, then a small lean focused team is the way to go. The issue with hiring 10k devs to build the core of an os is that just by definition, you'll have people starting the user land at the same time as the core and now you have UI project managers dictating ABI interfaces at the same time as you're trying to make your core OS.

A large dev team for something that complex it the classic case of the mythical man month. Some things just need to happen in series, and some things just need to happen first.