Amazon's growth is a systemic risk to Seattle

I know Facebook's second largest office is here, Palantir just opened an office, and it seems every week or so I hear of another SV company opening a satellite. That's not to say this isn't a real problem, but Amazon's success (and how frickin' awesome Seattle is) is attracting other people to cushion the blow.

An important note here is that the revenue sources and the markets for these are very diverse. The 5-10 notable companies with notable presence have probably 5-10 mostly discrete markets driving their growth.

Silicon Valley, on the other hand, has hundreds of companies but 90%+ are entirely dependent on ads (or the ad bubble, depending on which side of that fence you're on).

Any thoughts on how this plays into the risk calculus?

Will the trend of working for many companies for shorter periods of time, and therefore perhaps moving more often, make certain city populations less stable than others?