Hmm, so was there no difference in pronunciation between ye (archaic second person plural, nominative) and thee (archaic second person singular, accusative)? If y is equivalent to th, how come we once apparently had ye and thee existing in the language, with different though related meanings, simultaneously? In the 17th century, would thou and you have been pronounced the same?
Methinks the notion that the authentic, period pronunciation of the initial y in such words would always have been like a modern th may be a considerable oversimplification.