I'm also not saying that there's anything wrong with these companies expanding their presences outside the valley. (I would very much welcome it), only that we're not seeing enough of it right now to contradict the notion that these companies are still very much centralized. Maybe real estate prices and competition for tech talent will eventually force them to properly decentralize. But I could also see a scenario where those companies could put their might into play to overcome that resistance-level and allow them to continue growing while remaining centralized. It's simply too early to tell which way this thing is playing out.
What are you basing this on, and what would be enough ?
4.3%, if indeed happens that way, would be a substantial portion. They already have significant presence in NYC, Seattle/Kirkland, London, Zurich, LA, Hyderabad, Dublin, Tokyo, Austin, etc. A few thousand here and a few thousand there add up quickly.
EDIT: Added a few more that I remembered.
"This information is most likely buried in Google’s annual report — the last one I could dig up is their 2015 fiscal year end report. According to the paper published by Google Inc and Alphabet Inc, there are 61,814 full-time employees with 23,336 in research and development."
Bear in mind R&D isn't pure developers. It also includes QA and project/program management roles.