in the LXX, the purpose of the coming prince is to be used for the destruction of the city and the sanctuary by the anointed. That is his function per the Greek grammar.
interestingly enough, in the LXX, the prince who is coming cannot be the agent of verb “establish” based on the grammar.
You are missing the point that the prince is still the Messiah.
Your source is a very bad English translation of the LXX. It is not a specific but generic use of the word "it". The "it" is still the Messiah the Prince. Jesus is God. Other translations use the word "he". God is just the person doing the action through the people of the Prince (also God) to come. Jesus as a person is not doing the action, so did not come in 70AD.
So you have to figure out who is the "he". Since the LXX does not even include the original Hebrew about "the people of the prince", as "it" is not true to the original Hebrew. That is a bad translation into the Greek.
"It" is not a good rendition of "the people".
"He" is not a better rendition of "the people".
Why does the LXX change the people to an assumed God? If they are going to imply God, why use "he" or "it"? If you are going to remove "the people" from the original Hebrew, then use a word that defines God clearly.
The emphasis is still on the people who destroys. The LXX just changes that to "it" or "he", but still not the prince to come.
God allows the people to destroy their own city. God is doing the action through those people. So the LXX still does not prove the prince is a Roman. The prince of God is still Jesus the Messiah, and still to come. The prince of "the people", the prince of the "he", or the prince of the "it", is still Jesus no matter what version you use. And Jesus is never the emphasis of the one doing the destruction. So Jesus did not come in 70AD. God was there, and the people of God were there. Jerusalem was destroyed. The Romans were involved but not necessarily mentioned in Daniel 9:26. Josephus was there. He was not mentioned in Daniel 9:26 either, except he was a Jew one of those "people". He claims he was not one involved in the destruction, itself though.
No one should deny God was involved. The issue is always was Jesus referring to 70AD, or His literal actual Second Coming in the OD.
Obviously Jesus is both the Messiah and the Prince to come. Jesus was not present as either Messiah or Prince in 70AD. Those present were the people or God based on one's translation. Obviously both were there. The translation states one or the other. And neither the people nor God are the Romans, who were also there, just not mentioned at all in Scripture.
We talk about faith. If Daniel declared the Romans would be there in the first century to crucify Jesus and destroy Jerusalem, by name, ie literally including them in the text, would history have been different? Would people have concentrated on the Romans to prevent prophecy from happening?
Why today, imply or force the Romans into Daniel 9? Would it make a difference? Can we today change the past?
Rome was the 4th kingdom, the two legs of the image. Yet God allowed His own chosen people to totally reject Jesus and God allowed for the fulness of the Gentiles to happen. Why change history and God's Word to suit human theology? The 70 weeks are not finished, so why today are many deceived just like the first century Jews and declare the 70 weeks finished? They rejected the 70th week in the first century. Now today some reject Jesus is the King/Prince to come to finish the 70th week at the Second Coming.