Except that isn't what happened. Mark built Jesus out of the letters of Paul and various pagan-jewish works. Matthew copied Mark but really liked the Pentauch. Luke copied Matthew and tried to shore up the apostles. John took all three and some local Messiah stories.

None of these people were eyewitnesses, none of these people even talked to eyewitnesses. A copy of a copy of a copy with no original.

This isn't true

‭‭Matthew‬ ‭5:17‬ ‭ESV‬‬ Jesus said “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them.

The old testament is still quite relevant. There's also quite a bit more than the golden rule in the new testament.

For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.

Matthew 5:18

The golden rule is the only thing that really matters.

Yeah that thing that was articulated 5 centuries before Jesus supposedly lived and was known in Aramaic at least a century prior by Rabbi Hillel.