You're missing the point, it's not Switch vs Steam Deck, or even Switch vs Steam Deck and PC Gaming Handhelds.
It's Switch vs. Steam, which is available on desktop, laptop and handheld form factors and in the future likely a lot more (see box86/FEX, Chromebooks, Deckard). Valve doesn't care where you buy your games, they care that you buy them from Valve.
Valve don't need Steam Deck to become as big as Switch, Steam as a whole already is.
When you buy an Xbox game, you're buying a PC game. Both Sony and Microsoft's first party output are available on Steam. You can run Microsoft Office on an Xbox, the Steam Deck by default has the UX of a console. Sony and Nintendo are the only companies sticking close to their roots, but as mentioned, even Sony is letting that slip. "Console wars" is a misnomer, an old label inherited from the Sega v Nintendo days, it's not that simple anymore.
Windows isn't the "winner" because Windows isn't an app store, Microsoft don't get a cut of every game sold on Windows, but from sales through their store. That's the revenue stream that matters, moreso than how many Xboxes or windows licenses they sell.
You're not wrong that a lot of people don't see things this way. But they, particularly regulators, probably should.
It's Switch vs. Steam, which is available on desktop, laptop and handheld form factors and in the future likely a lot more (see box86/FEX, Chromebooks, Deckard). Valve doesn't care where you buy your games, they care that you buy them from Valve.
Valve don't need Steam Deck to become as big as Switch, Steam as a whole already is.