I understand where you're coming from with this, but i disagree.
The steamdeck as it is right now, doesn't have to do that. It's not meant to be a PC. It's meant to be a handheld console that can also function as a PC. It's main purpose is to open steam handheld, and play games from your steam account.
The people who would use this thing as their main PC would have the technical know-how to configure it to boot into desktop mode without steam open.
The OP was about the stReamdeck, but @Piratepower42 started talking about the steamdeck because I guess we mentioned steam earlier?