At most it's just brain drain from around the world (and around the U.S.) due to capital concentration NOT talent concentration.
There's this belief that anybody who isn't willing to move to high COL metro couldn't cut it there because having a worldview outside of living in SF or similar isn't something that can be fathomed.
Brain drain implies relative talent concentration - are you sure this is what you mean to say?
Look - I hate SV and don't live there. The reality is unavoidable: for software, the best talent lives in the Bay. It's not that deep.
I'll go with the world.
Microsoft has brilliant engineers and is in Seattle, for example.
What about Tencent? and things like HK's HFT
What about competitive security? here's huge variety in countries e.g 2 very strong teams are Poland based