It's definitely best as a companion to a full gaming pc. You get to play games when you are travelling, staying over somewhere else, or just want to hang out on the couch whith your partner while they watch something on TV. The suspend-resume feature is also amazing.
It can do a lot, I love it for what it is but it also has limitations. Games are just better on a larger, higher refresh screen if you have the option (I'm particularly picky about tight fov settings). Also some games are just easier with keyboard and mouse. Slower, more casual games you can fully enjoy on the steam deck.
So the pc is not going away but the sd is a great accessory in a sense.