Didn't Jehovah say in Psalm 89:34 "My covenant I will not break, Nor alter the word that has gone out of My lips." Would not keeping His covenant to the Fleshly Israel constitute a "break" in that covenant?
On what basis was the covenant made? It was on the basis of Israel's obedience to the commands and laws of their God, which they willingly entered into of their own volition. (Exodus 19:6-8)
God kept to his part of the covenant by producing their Messiah as he promised Abraham. And once he had done so, he gave Israel exclusive opportunity to accept Jesus as "the one" for three and a half years.....but, not only did the majority reject him...they orchestrated his murder. (Matthew 27:25) They cursed themselves with his blood.
So on the night before his death, Jesus instituted a new covenant which made the old covenant obsolete. This was prophesied by Jeremiah.
Jeremiah 31:31-34...
“Look! The days are coming,” declares Jehovah, “when I will make with the house of Israel and with the house of Judah a new covenant. 32 It will not be like the covenant that I made with their forefathers on the day I took hold of their hand to lead them out of the land of Egypt, ‘my covenant that they broke, although I was their true master,’ declares Jehovah.”
33 “For this is the covenant that I will make with the house of Israel after those days,” declares Jehovah. “I will put my law within them, and in their heart I will write it. And I will become their God, and they will become my people.”
This is spiritual Israel, who were to "become" Jehovah's people...not the covenant breakers who supposedly, already were.
Would not substitution Fleshing Israel to a Spiritual Israel constitute an ALTERation of said covenant?
No...for all the reasons just outlined. God had fulfilled his part of the covenant and it was finished.
Did not Jesus Himself say to the Samaritan woman "salvation is from the Jews." (John 4:22)
Salvation did begin with the Jews, but by no means was it to finish with them, as the covenant with Abraham had stated....
Genesis 22:17-18....to Abraham God said....
"I will surely bless you and I will surely multiply your offspring like the stars of the heavens and like the grains of sand on the seashore, and your offspring will take possession of the gate of his enemies. 18 And by means of your offspring all nations of the earth will obtain a blessing for themselves because you have listened to my voice.’”
"All the nations" were to receive a blessing from God because of God's promise to produce the seed. Jesus did not come to die for just the Jews...he came to die for all obedient mankind.