ABIs have always been platform-specific in that sense. C ABI is beneficial in that on any specific platform, it's interoperable - which is good enough, because native code has to be compiled for a specific platform, anyway. In your specific example, I don't really care how "struct foo" is passed, so long as all shared libraries compiled for that platform agree on how it's done. And we have that on C level today on all major platforms.

Sure, there are compiler switches and language extensions that can break the ABI if you use them. But, well, you don't have to use them (at the interop boundary), and neither do your API clients.