Needless to say, the issue you raise is a controversial (and potentially volatile) one, and the position Orthodox Christians take can easily be misperceived as anti-Semitic. The problem is brought into rather stark relief by the text you cite from Hebrews 8:
“Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah…. In that He saith, A new covenant, He hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.”
As is clear from the context, the “old covenant” to which Saint Paul refers included an obligation to perform a ritual animal sacrifice, and man’s fulfillment of his side of the covenant therefore required a place ritually set aside and marked out for the purpose of such sacrifice—in short, a templum or “temple”. So the question arises: When, after the destruction of the Temple in Jerusalem in A.D. 70, there was no longer an appropriate templum, and thus no sanctified space for the required sacrifice, did that covenant remain in force, or is it now “made old” or “obsolete”?
Of course, Judaism continued, and continues today, in the synagogues, but compared to what took place in the Temple, this continuation—it could be, and has been, argued by traditional Christians—is but a “copy and shadow” (Hebrews 8:5), just as animal sacrifices were themselves a “copy and shadow” of the sacrifice on Golgotha, and even as Golgotha in its turn was a “copy and shadow” of the heavenly Sacrifice (Revelation 13:8). So the question becomes: How many shadows does it take before the Light no longer illumines? How many times can you copy a thing before it is no longer legible?
I don’t claim to have the answers. But it’s precisely such highly charged questions that lie behind the issue raised by certain Traditionalists: namely, whether—and if so, to what extent—Judaism remains a fully operative Path. One thing I do know: if the Liturgy of the Catechumens were the only Liturgy—if there were no anaphora and if Christian initiates were no longer able to receive the Eucharistic Mysteries—Christianity would cease to be a fully operative Path. I’ll leave it to others to argue by analogy, if there is such an analogy, as to the state of contemporary Judaism.