The Steam Deck can play more than steam games. I usually keep about 30-40 Steam games installed, but most are really light little games for casual play with just a couple of large AAA games at any given time.
But outside of Steam, the Deck can emulate up to PS3 and Switch games. And theres not really a good cloud-based solution without having to switch into Desktop mode. So I have my usually about a dozen PS2 games, one or two PS3 games, and my whole PS1 library on the Deck. Then every couple months I’ll go into Desktop mode and delete what I’ve beat and copy over new games from my gaming PC.
I’m also not a dock user. Maybe I would be if I didn’t have a gaming PC, but for me the point of using the Deck is to not be attached to anything. Otherwise an external HDD might help. I’m thinking about eventually building a NAS: I’m not sure if I could have games live there and still playable on the deck though?