S&B existing and being what it is, effectively makes it the Source 2 SDK, although it's not from Valve. But fair point Source 2 isn't licensed to others, I think the expectation is that if you wanna build a Source 2 game, you have to use S&B. At least for now, who knows what their ideas and ambitions really lie.

>They barely have their toes in the middleware game anymore.

Well they do have Steam Audio but yeah I agree. I think Epic is much better in this space, even though its only source available in practice they do a lot to support engine modifications and also accept external PRs. I think Valve has a lot to gain from open sourcing Source 2 and they should realize how important modding was to their initial success. The issue is now they can just print money with Steam so there is no need to invest in modding support.