God established an eternal covenant with Israel
Your argument raises one of the most fundamental questions in the relationship between the Torah and Christianity: if the God of Israel declared His covenant, commandments and promises enduring, by what authority can a later religious system claim that these institutions have been fulfilled, transformed, superseded or abolished? If we take the Torah as the governing authority, rather than assuming the New Testament is authoritative from the outset, the question becomes particularly challenging for Christian theology. The crucial issue is not simply whether Christians believe in Jesus as the Messiah. It is whether their claims about him can be reconciled with what the God of Israel already declared.