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".
One of the link of past discussions was from Apr 2018 and discusses it. At that time GCC -fplan9-extensions support was too recent (gcc-4.6) to be considered. https://lore.kernel.org/lkml/20180419152817.GD25406@bombadil...
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.