It will be interesting to see what kind of people go for something like this. I doubt many PC gamers want to use a device like this for PC games when they have become accustomed to a keyboard and mouse.
I think the $399 version of Steam Deck with 64GB of storage can be popular with buyers who only own a slower netbook, tablet, low-end smartphone and older/slower full size desktop computer which are not fast enough for most PC games.
I think the $529 and $649 version of Steam deck with more storage maybe less attractive to many buyers because buyers can get a used gaming computer for a similar price on eBay, and use a game console controller to play games if they don't like a keyboard and mouse for gaming. There are also apps which let you stream PC games to a tablet, smartphone, and settop box like Steam link and Android TV boxes.