I agree, the oracle scripts are too mature to come out of nowhere, it has to be derived somewhere and some time earlier than Shang dynasty. It is almost certain that it is from something around the time of Xia dynasty (or a culture than a state). It is just that the archeological evidence has lost long time ago because unlike ancient Egyptian culture which is primarily built on stones Chinese cultures were primarily based on wood or ceramics back then.
While it's certain that there were some form or forms of precursor script for the Oracle Bones script, that is no proof of a unified state as described by the Xia dynasty.
However, one cannot discount oral traditions simply because no written accounts have been found. Many cultures did not have a written script until well into the middle ages, it does not mean their history is "fictional".
We know of Xia dynasty through written accounts that date back to the Zhou dynasty, around 1000 years after Xia. That is not such a long time that oral traditions would be warped beyond any semblance of reality. The argument that Zhou invented Xia to justify overthrowing Shang is weak at best, and smacks of trying to justify a preconception.
Personally, I believe the Chinese myths, up to the Yellow Emperor, have historical basis.