Same what FreeBSD has done: keep developing Open Source OSS. One implementation going non-free doesn't affect other implementations of the same API.

Didn't they do that by developing ALSA OSS emulation? That is effectively another implementation of the same API.

Libalsa is there in FreeBSD ports, but it's only for backward compatibility with Linux, and it's only userspace parts. The kernel implements OSS API.

I mean Linux, it does the same but reversed: the kernel implements the ALSA API, and libaoss is provided for backwards compatibility in userspace with OSS. What else should they have done? The ALSA API is not the same as OSS and has a different set of features.

Feature set doesn’t really depend on the API, FreeBSD implements all that functionality despite using OSS.

It sounds like Linux and BSD both have an implementation for both APIs, only implemented in different ways? If so, I don't much see where the problem is with this solution.

They don’t. And the problem is that replacing a popular API with a proprietary one, like ALSA, creates a lot of work for everyone, for no reason other than NIH.