I feel like that's irrelevant, because the same logic can be used to justify having Source2 open as well.
After all, what value is there in keeping the game code private at all? The only things I can think of are a) anticheat, b) NDA'd 3rd party code, and c) protecting important secret sauces made by the company itself
a) and b) are both solved by simply not releasing those portions of the code, while c) is a bit moot. Any competitor can already (legally) RE the binaries and recreate any juicy proprietary algorithms, unless they're patented. But in that case they would be protected even if the source was released so it's still the same situation.
I'm not fully up to date but I'm under the impression that Source 2 is currently used by their modern games. I don't think that's the case for Source 1. The same logic wouldn't apply.
>After all, what value is there in keeping the game code private at all?
Pretty much what you said. It's all down to cost-benefit at the end of the day and of they think there's more cost (perceived or otherwise) to releasing source 2 code they won't bother.
Far more likely the community kicks up a huge stink about how Valve released a crippled engine.
There are maybe four still in development and due hopefully this year, plus Apex Legends is still going strong.