Valve does not have a monopoly by any definition of the word, especially the legal definition. They don't have a majority of the business because they buy out the competition or use their position to drop prices to a level that others can't compete with. They have a majority of the market because they provide a better service than the competition and have been doing it long enough to have developed a cultural gravity in the same way that Xbox, PlayStation, and Facebook and Twitter have.

Well, OK, let me ask this a different way then...

How could they possibly distribute a device that was end-user ready with any less control?

They went out of their way to make the Deck user-repairable and partnered with iFixIt to make replacement parts and documentation easily available. They built in a simple way to switch to the Arch desktop so the user could easily access non-Steam applications or take control of the OS if desired. I don't see how the device could be more open to end-user control and still ready-to-use out of the box.