Now the reasoning isn't present in the patch but it probably is because they want step increments and -fms-extensions is a small-ish first step. Maybe -fplan9-extensions could make sense later, in a few years.
Plan 9 extensions would only require enough examples to justify and might not take years. Though your taking years assessment would be right if there's a dearth of kernel spots to add up where automatic pointer conversion for anonymous fields, or using the typedef name to access them, offer some improvement, not necessarily even a huge improvement.
Since with the Microsoft extension, it was just waiting until enough examples were woven into the discussion to overcome the back and forth that was preventing "biting the bullet".
Can you confirm whether or not anonymous member structures originated with the Plan 9 C compiler? I know I first learned of them from the Plan 9 compiler documentation, but that was long after they were already in GCC. I can't find when they were added to Microsoft's C compiler, but I'm guessing GCC's "-fms-extensions" flag is so named simply because it originated as a compatibility option for the MinGW project, and doesn't by itself imply they were a Microsoft invention. GCC gained -fms-extensions and anonymous member structures in 1999, and MinGW is first mentioned in GCC in 1997. (Which maybe suggests Microsoft C gained anonymous structure members between 1997 and 1999?)
Relatedly, do you know if anonymous member unions originate with C++, Plan 9 C, or elsewhere?
The same extension can be enabled with `-fplan9-extensions`, might be more appealing to some!