That's a different discussion and it's hard to untangle and weigh all of the effects. But it's worth noting that the % of people working multiple jobs has actually decreased with the rise of Amazon.
That's not to say it's casual. Maybe it would've decreased more sans Amazon. Who knows.
Anyway, the point was simply that it's not only/mostly engineers who derive value from their existence. It includes every Amazon customer who saves time & money - especially those who have too little of both.
It also includes every person who lives in a city whose tax base has been eroded by Amazon, so those "hard working single moms" send their kids to shitty schools that will perpetuate the cycle of poverty.
For tens of millions of working poor who live in apartment complexes, Amazon provides absolutely zero benefit since they cannot order anything bigger than a kumquat without risk of it being stolen off of their porch. The office? The office doesnt exist or is closed during the hours they are working - and UPS wont deliver there anyways a lot of the time.
Low prices aren't free. Some of it came from efficiency, the rest comes from simply paying labor less.