Just pulled up their filings[1], and I'm almost certainly misreading it, but in Q3 2017:

  NA Retail Revenue: $25,446M
  NA Retail Profit:  $   112M
  AWS Revenue:       $ 4,584M
  AWS Profit:        $ 1,171M

Pedantically, it seems like AWS is not Amazon's primary source of revenue, but is the primary source of profit.

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[1] https://www.sec.gov/Archives/edgar/data/1018724/000101872417...

Inaccurate wording on my part. AWS is amazon's primary source of "operational income" (i.e. raw revenue minus operational costs) these days. $1.17 billion for AWS vs. $0.35 billion for Amazon as a whole, for Q3 2017.

https://www.cnbc.com/2017/10/26/aws-earnings-and-revenue-q3-...

AWS is also the primary driver behind amazons stock price.

Within a few months or years amazons shopping marketplace could become eclipsed by another. Consumers can switch brands quite quickly if they see a better service elsewhere.

AWS on the other hand is very hard to migrate from. Can you imagine the investment necessary to move a large company off AWS infrastructure, when you have legacy and unsupported services that nobody knows how they work, depends on intricacies of amazons managed services, and have no way of doing zero downtime data migration?

Basically, Amazon can gradually increase the price of existing AWS services, while lowering the price of all new services to attract new customers. (Think "From 2019 we will be releasing S4, the new cheaper data storage system!").