Kinda misleading.
First of all, games do not have to be on Steam & launch through Proton to be able to run on Linux. Wine hs gotten extremely good too, even if it may require a bit more tinkering in comparison.
It's also not like this because of the Deck. Proton has been on a good run for several years now, which was very much evident based on the stats on ProtonDB. The Deck helped more with popularity & spread of Linux, rather than actual compatibility.
And those unsupported titles are almost all competitive multiplayer games. Regular multiplayer titles that are mostly PvE focused work usually fine under Linux.

I suspect the Steamdeck and Steam machines were the main motivations for Valve to put as much resources behind Proton as they did.

No they did it for the money, steam deck isn’t a good handheld, it can become outdated in a few years, Linux hacks is a real possibility, etc.

No real reason for Steam deck existence other than money.

Well, I mean… of course, yeah? They’re a private a corporation, they want to make a profit on anything they do. But the idea is the Steamdeck with Linux on it is worthless if it doesn’t play games, hence why so much money was poured into Proton to make the Steamdeck a desirable and profitable object.

Saying it’s not a good handheld because it will become outdated is like saying every laptop in existence is bad because it will eventually become outdated. You sacrifice upgradeability for portability.

Technology can be upgraded and Valve chooses to make the deck a worst version of handheld. Just look at other handheld, less problem with holding and more gameplay experience. Proton doesn’t solve the linux gaming issue, companies are more willing to lose the Linux platform for protection of their games.

Gaming in Linux shouldn’t become this bad, one company controls the entire reason for the Linux platform, there should have been more if not better ways.

The other handhelds cost more, and usually have worse battery life.

Proton has absolutely solved the chicken and the egg problem. There’s a big enough audience on Steamdeck that many devs of online games have flipped the switch to allow their anti-cheat to work under proton.

there should have been more if not better ways.

Linux gaming was stagnant for over 20 years, what would you have done differently?

The other consoles are built for gaming and offering features that Valve hasn’t even announced for such. Moreover the steam deck being locked in the Steam ecosystem doesn’t help the “solved problems”. One company that is known for greedy developers is in control of Linux Proton.

It’s clear what should have happened, the steam deck shouldn’t have existed. Valve the billionaire company should’ve offered better alternative or find better hardware. We have computers in the size of a credit card and they decided a brick with unnecessary specs with uneven screen size is the best? They clearly didn’t care anything but cheap hardware and software that is free. The steamOS isn’t open source and you people are letting Valve make their money on free software.

you people are letting Valve make their money on free software.

Making money on free software is absolutely fine. Even GNU, perhaps the most hardcore free software group, has this to say:

You may have paid money to get copies of a free program, or you may have obtained copies at no charge. But regardless of how you got your copies, you always have the freedom to copy and change the software, even to sell copies.

There’s absolutely no problem with Valve profiting from free software, the only obligation is that they share their modifications to any free software they use with their users. Here’s their fork of Proton, and here’s an article about the rest of the software stack they have modified (i.e. they’ve made lots of changes to the graphics stack, as well as various other parts that are important to their use-case.

So it’s not like they’re taking and not giving, it’s a two-way process where they and the community both benefit from using free software.

The other points have been well covered here, I just wanted to point out that making money is not a bad thing.