While technically true at a literal level, “proprietary PC” gives the impression of closed systems like OEM laptops. “Just” has a bit of a diminutive tone to it.
While Steam itself is proprietary / closed source, the Deck’s Linux OS foundation is open source. Despite being developed by Valve, Proton is also open source and is a massive boon for the viability of Linux gaming.
OEM parts are directly available from an official supplier along with detailed repair documentation - things stop short of schematics, which I look forward to seeing change. Third party parts (screens, face button tactile upgrades, etc) are available and able to function uninhibited. The device is clearly designed with user customization and reparability in mind.
With that said, the deck feels so much less proprietary than the PC ecosystem.
Not saying that you are wrong or that people are right to be downvoting you, I just think your phrasing comes off a bit harsh which led to negative impulse reactions. I know that my initial response was negative until I sat and thought about what you actually meant for a minute.