Bringing a game to a new platform is phenomenally expensive - 7 or 8 figures for a large game. It’s not just the port, it’s testing, it’s updating docs, it’s updating support people. That is money someone has to invest up-front, so the people with the money need to know they are going to get a reasonable return on that money compared to building a new game

In this case, it really isn't. The platform they need to support is Windows before and after. No change there.
The only actual change they need to do is set a setting in their anti-cheat middleware to allow Proton and distribute the required binaries. Obviously a bit of QA that that part actually works.
The rest is up to WINE/Proton/Valve and supporting systems and 99.9% of that should already work. We as the broader Linux community have full control over those, so there's no further input required from the game dev after that.

It's maybe 1-2 dev days, perhaps a week. That works out to 3-4 figures, maybe 5. I suspect that'd be offset in the first 5min of even just announcing Linux support.