IMO, Capital and Talents are the two major factors in determining whether or not a location has a potential to have a good and sustainable tech scene. You'd have to have good supply and demand for both to sustain a healthy tech scene, and the driving factor between the two is capital, not talents. The exodus of talents in SD is a symptom of weak local demand (not because of, say, not enough restaurant options); which in itself is a symptom of weak supply of capital. Any place's scale of tech scene reaches to a local equilibrium when the supply and demand for both factors levels up. Following that logic, San Diego seems to be going where that local equilibrium is, given the current scale of capital available in the region. But the fact that more VCs are looking at places outside of the bay may bring some hope to SD, depending on whether or not SD will be "the chosen city".