You cant adapt to something if you have not the inherit quality that is required and cant have it.
A shikagami can not adapt into a human, it is at the end of day a Shikagami. If that what you implying was correct, which is not the correct reading of the word to begin with, Mahoraga would have adapted to desummoning and disappearing, or even being bound to a contract, which it clearly hasnt despite being summoned multiple times.