It is my opinion that Amazon will live or die by this decision. Seattle is the fastest growing (per capita) city in the United States and already has had (for decades) a housing, traffic and parking problem. The plans that Seattle has to expand public transportation do not include plans which will help the chokehold situation in SLU.
Currently Amazon only works because: the cost of housing versus the income from Amazon is balanced with the insane hours they demand of their employees and the traffic that is only helped by the odd hours that employees end up working. As the price of housing increases, especially in those areas with easy access to Amazon HQ, and as the traffic situation becomes worse, and as Amazon burns out both its reputation and potential long term employees, the long term prospects of Amazon's HQ IMHO are not very good. The more employees are squeezed and the less they are compensated, the more the pressure to work must come not from compensation but from political pressure - but I think other opportunities in the area will drastically outperform Amazon in terms of compensation and work-life balance in this world, so I expect Amazon to falter.
I have nothing but praise for Amazon and as time goes on I find myself more and more simply ordering things from Amazon even if they're available locally.