Assume the Trinity is correct. Why can't people be saved if they are mistaken and believe it is false? God can't save people, despite their ignorance? If the Trinity is correct, why would God save people (Noah, Elijah, Moses, etc.) who did not understand his Trinitarian nature?
The Jews that knew Christ accepted Him because they knew He was the Messiah. The Jews after the doctrine of the trinity don't because they know he isn't God the way the trinity describes Him.
Current Jews. What of them? They deny Christ is God, AND they deny Christ is the Messiah.
If the doctrine is from man, God can show them mercy. If the doctrine is from Him, He cannot.
You propose to limit God and His mercy? Bold, my friend.
What do you think of Christ's command to baptize in the name of the Father, the Son and the Holy Spirit? We should baptize in the name of God, plus some other things that aren't God?