When I need to make 10,000 cheap and fast, yes, I'll most likely be making a pilgrimage to Shenzen. Totally agree with your point. But not every startup (especially low-volume, high-price, high-touch b2b) needs to focus at the start on making 10,000 units cheap and fast.
So, let's say now you need 10,000 of them. For 1/4 the price. Delivered next month.
That's where Shenzhen starts looking really nice.
Especially if you've already got reliable contacts there, like Bolt does with Dragon Innovation.