"Who said Saul failed?"
God. God said He was sorry He made Saul king, you disagree?
God didn't say Saul had failed. God said He was "greived" He made Saul king. Please show me the verse where it says Saul was a failure.
Here is a major problem with your perception that God was surprised or taken unaware by Sauls disobedience . And so he was grieved and changed his mind.
We know that God had foreordained the plan of salvation from BEFORE the foundation of the world. Jesus had agreed and the plan developed even before Adam fell
That plan Had Jesus as prophesied ad the Lion of Judah and the SON OF DAVID
Saul and his line were never the ordained ROYAL line for the Messiah . The Kings of Israel were never ordained to come from the tribe of Benjamin
Saul sinned, God is never pleased with sin, but God was not surprised by the sin . God had made Saul, it was god that had formed the character of Saul. God that knew exactly how Saul would react in a given set of circumstances , if His hand did not restrain him.
If the Lord had not revealed to Saumel a change toward Saul because of his disobedience (1samuel15:1-10) then Samuel would have had grounds for thinking that the Lord's character had truly changed. The biblical narrative of 1 Samuel 15 does not bring into question Gods eternal purpose to raise up Israel's king from the tribe of Judah and not from the tribe of Benjamin (Gen 49:8-12), instead it shows how he brought about his prophetic word that the Messiah would descend from Judah!