I begrudgingly trust microcode. The point isn't whether or not I do trust microcode, the point is I shouldn't have to - there's no reason for it to be so secretive
So we are to trust our entire security to implausible what if scenarios?
It has been shown that government agencies were ahead of public research in areas of (specifically) cryptography in the past, so it makes sense that they would still be ahead today (that is one of their goals too I think). There have also been instructions in processors that didn't make a lot of sense at the time, though I don't recall a specific example or what it was speculated to be for.
The only thing I think is fanciful is this being the primary reason to have microcoded instructions in the first place. I wouldn't be surprised at all if certain customers had access to special instructions via microcode updates.