It's not coming "via Proton", DLSS has been in the Linux driver since July 2020, this is expanded support to allow it work with Proton. We explained a bit more here, along with links to the work ongoing.
Proton is available in the normal Steam client, and has been for years now.
It also wouldn't be "part of the core Linux Steam Client Beta", since Proton is separate to the Steam Client. It installs like other games and applications, the work will all be inside the Linux drivers linked above and Proton itself.
For Valve, Linux is primarily (to my understanding) a huge insurance policy. Tomorrow is just as important to today. Valve has seen stiff competition against Microsoft with the MS store, and if Microsoft (who loves to follow everything Apple does) were to pull an Apple and Valve were to find itself in an Apple/Epic situation (Edited to add: regardless of who is right in Epic/Apple. I don't know.) then where does that leave Valve?
That leaves Valve with Linux. What Linux is today compared to what it was before Valve became so vested as pro-Linux around a decade ago is a night and day comparison from the singular perspective of a gamer. It is such a pro-gamer platform now.(plenty of room to go, but still) We have AMD making completely open source drivers, and of course announcements like this. Nvidia can see the writing on the wall. Linux IS a gaming platform of the future. In many ways it already is a gaming platform of today. It's here. This is now.
Even before Valve came on the scene, I was firm that if a game wasn't Linux compatible, then it wasn't a game I wanted to play. I utterly refuse to buy it. And many Linux users are the same way. This is another (however small you want to say it is, that's fine) reason is that Linux users are extremely loyal to the platform. Microsoft has simply bullied too many people and of the enemies it has made, they are enemies that it rightfully deserves including its former customers such as myself. You just can't bully your own customers. Not without some consequence and it's taken a while, but here it is.
This is partly why at the end of the day, Linux (again, however small you want to say it is, that's fine) is a growing platform. You can pick any 5 year window you want.
Five years ago, there were not as many Linux users as there are now today.
Five years prior to that(10 years ago) there were not as many Linux users as there were in the first five years.
Five years from now there will be more Linux users than today.
Now why WOULDN'T Valve cater to a clearly growing platform? This isn't MacOS where Apple has clearly shunned the gaming community by stiffing Vulkan and built Metal just to be different for the sake of being different.
There's a huge threat from Microsoft sitting there and the icing on the cake is that it's simply a fun platform which means that Valve's internal developers are probably loving every minute of it. Hey what's high internal employee morale worth? Just speculating.
To answer you simply, no, Gabe would not be better off putting time into a new Half Life because he may not have a platform to sell it on. But you have to look beyond today to see that.