I assume you understand that replacement theology heresy is part of mainline protestant, catholic and orthodox doctrine.
It probably started as speculation.
with common sense.
exactly how can a 2,000 year old consistent doctrine of the Church be in any way considered “heresy”?
Correct. Christ is the fulfillment of all the ancient prophecies, not their replacement. The technical term is typology.
Christianity is the New Covenant. But it does not replace and vacate the Old Covenant of Israel, it fulfills it. There are several place in the Bible where God tells Israel words to the effect that “My Covenant with you is forever.” The Jews may fall away and betray the Covenant, but God has promised that he will never abandon them. St. Paul (I forget exactly where) also declares that the Covenant is forever, although of course he converted, himself.
This does not mean that God does not welcome Jews into the Christian faith. But it does apparently mean that at least some Jews will remain, and will one day return from Exile and be welcomed back once more as God’s Chosen People.