Decompiling the microcode blob might run afoul of intellectual property laws. A clean room implementation needs to be done without reading any of the product's actual code.
Clean room design [1] comes down to one team reverse engineering the code and translate that to some high-level specification and let another team write the code based on that high-level specification. This still allows the use of a decompiler by the first team.
[1] https://en.wikipedia.org/wiki/Clean_room_design