This is the reason bezos thinks long term. because he can.
Some of those parts are:customer trust,the need for computing,fulfillment,delivery,payments and access to targeted eyeballs,all content will be digital, need for marketers(affiliates).
Serving as a leading infrastructure platform is a great way to achieve this.It gives you revenues , it puts you under customer pressure and i think the new market dynamics works better for amazon also as an integrated retailer, by letting amazon control prices and service levels and limiting the platform, by decreasing competition to a small number of companies in the industry and by pulling startups towards them.
Now combine the (small?)benefit of each platform gives you over competitors, together, and you got a big competitive advantage.
On the other hand, having a great platform let's you profit and learn from the things that change fast.