The API/ABI structure of every modern OS is a hot mess, that has nothing to do with syscalls not being files, and everything to do with the way the systems have

The API/ABI structure of every modern OS is a hot mess, that has nothing to do with syscalls not being files, and everything to do with the way the systems have been extended over and over again for decades. Making a simpler API is a great idea, and from you comment I guess they mostly succeeded, but they did not have to go down the everything-is-a-file path to do that. What role this played in lack of adoption depends on what alternate course of action you compare it to. A non-file-based rewrite of the API would likely have had approximately the same result, in any case a good compatibility layer might have made a big difference.