The very verse you provide Matthew 24:14 supports my argument that Matthew 24 is speaking to a future generation than that of the disciples
Had the gospel been preached to all the world nations in the disciples 1st century AD Generation "No"
You seem to have me confused with Preterits?? I've never said the Gospel of the Kingdom of God had been preached unto all the world within the first century. Even though Paul seems to indicate the Gospel had been preached unto the all world known then, we know that could only be the known world, otherwise why continue to preach the Gospel? We should look at "all the world" as being the same world that should be taxed by Caesar Augustus. The Gospel of the Kingdom of God must be a witness to all nations of the world, and since many nations have been born long after the first century AD, we know the Gospel could not have been a witness to nations that were not in existence then.
Luke 2:1 (KJV) And it came to pass in those days, that there went out a decree from Caesar Augustus, that all the world should be taxed.
Both Preterits and Futurists make the same mistake of trying to force the prophetic words of Christ spoken from the mount of Olives to fulfillment within one very specific period of time. The words Christ spoke are not to be limited to any one specific time 70 AD, nor are His words are not to be limited to the very end of time just before Christ returns the second time. Christ is speaking to His Church on earth about things His disciples will be called to endure by faith as they are sent unto all the nations of the world to preach the Gospel of the Kingdom of God.
And one thing is without ambiguity, Christ will not come again until the Gospel of the Kingdom of God has completed the task for which it is being sent, that is to complete the spiritual Kingdom of God in heaven as people from all nations hear the Gospel preached in the power of the Holy Spirit and come into the spiritual Kingdom of God when they are born again through the Spirit of Christ in them.
What was not understood before the Spirit had been sent to man after Christ ascended to heaven is how the Kingdom of God is entered, and how man might enter that which is not visible but is within you. That's why these first century disciples of Christ needed a sign that the spiritual Kingdom of God was not being preached in vain, but that it was indeed accomplishing the purpose for which it was sent.
Christ told them how they would know the Gospel of the Kingdom of God was being built as it was preached as they would see people come alive spiritually through the power of God. There would be spiritual life seen in the fig tree (Israel) and all trees (Gentiles) where before there had only been darkness through unbelief and fear of death.
The nation that had been cursed by Christ to never have spiritual life within again, through a remnant of Jews there would be a remnant according to election of grace (Ro 11:5) to show they too had come into the spiritual Kingdom of God when they heard the Gospel of the Kingdom preached and by the power of the Holy Spirit they repented and turned to Christ in faith believing in Him.
These first Jewish Christians were the beginning of the "chosen generation" (1Pe 2:9) or "this generation" joining with the disciples of Christ that would expand to be an innumerable multitude and would never pass away, not even after all these things Christ spoke of would be finished.