Well, an Amazon worker, since robots help bring him the stuff and a computer micromanages his every action, is probably 5 times as effective at moving product as a retail stocker is. So +20k seasonal jobs = -100k shitty retail jobs elsewhere.
How in the world is this doom and gloom?! This is fantastic, it means that the concept of having goods in a warehouse that are mass-shipped by carriers like UPS is more efficient than a physical store that consumes a much more space, uses power/AC/light 24/7 and where small groups of shoppers inefficiently load a few bags of groceries into their gas-guzzling automobiles.
This is progress, and yes, when the printing press came out, scriveners and monks were SOL. And the ratio of number of jobs replaced was probably far higher than 5:1. We as a society should absolutely help them find something else to do and provide a social safety net if they fail, but that's a far cry from saying it's doom and gloom that we are processing goods faster and cheaper with less environmental impact.
Doing more with less is the magic sauce that makes any of this shit like a specialized economy and your computer even possible. In 1840, 70% of Americans still worked in agriculture meeting our basic needs for food. Today it's 2%, and we've got (way too much!) food for everyone and the other 68% went on to do something else productive (well, in 1840 we didn't have a social safety net, so that's bad for those that didn't, but on the space of generations...).
So yeah, increased productivity, lots of churn, doing things smarter instead of with more labor and helping all people that want to work find a non-make-work thing to do. Much less doomy and gloomy.