So you're saying he WAS just kidding.
Jeremiah said the new covenant will see the law of God written on the hearts...so no, Jesus wasnt kidding when he said he didnt come to abolish the law. He came to show people how to put the law on their hearts....and in that way, they could enter the new covenant.
And the New Covenant described in Jeremiah, which is only for "Israel and Judah" says nothing about the old Law being dissolved as you seem to agree here, rather it says the SAME LAW will be written on the hearts of the believers.
but Jeremiah says it will NOT be like the old covenant.
31 Look! There are days coming, is the utterance of Jehovah, and I will conclude with the house of Israel and with the house of Judah a new covenant; 32 not one like the covenant that I concluded with their forefathers in the day of my taking hold of their hand to bring them forth out of the land of Egypt, which covenant of mine they themselves broke
The new covenant was not going to be a copy of the mosaic law...it was going to be different. Gods laws were not going to change, but the type of covenant would change.
And how could it not, for example, Christ did away with divorce and polygamy....the mosaic law made allowance for such things. Christ did not. So there is already one change that he made which was different to the mosaic law.
This is why its a 'new covenant'... it has different conditions attached to it.