It's clear from those letters that Amazon is not an experiment, but rather a flawlessly executed plan about how an online retailer can exploit their inherently better capital flows to grow very quickly.
It's almost the opposite of the modern startup approach.
Not: "We need to borrow money and take investment to get big and dominate the market".
Instead: "If we drive our costs down and focus on fast fullfillment or orders/inventory turnover, we'll have all this spare money sloshing around to reinvest, and happy local customers. Therefore we can't help but get big.".
(There's a lot more to it than this.)
The first letter shows that they already understood the economics of web retail very well in 1997!
Accusing them of lacking pride is also definitely missing the mark. Amazon are optimizing customer satisfaction and rate of fullfillment. They enter all these other markets exactly because they are proud enough to believe they'll win. Amazon wouldn't enter a market aiming to be 2nd best.
I haven't gotten to the present yet, so I didn't know that Amazon had opened high-street retail shops. The earlier letters said that they wouldn't do that unless they figured out how to make it scale faster than their expenditures, like their online business.