It is totally relevant. You are trying to say I am reducing this down to an "allegory". I'm not. I'm saying this promise was fulfilled with Christ who now sits on a very physical throne, the throne of David.
Isa 9:6-7 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counselor, The mighty God, The everlasting Father, The Prince of Peace. Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth, even forever. The zeal of the LORD of hosts will perform this.
Mar 12:35-37 And answering, teaching in the temple, Jesus said, How do the scribes say that Christ is the Son of David? For David himself said by the Holy Spirit: "The LORD said to my Lord, Sit at My right hand until I place Your enemies as Your footstool." David therefore himself calls Him Lord, and why then is He his son? And the great crowd heard Him gladly.
Peter's remarks in Acts 15:
Christ reigns right now. He hasn't been sitting on His hands for 2,000 years waiting for the "right" moment. He doesn't need a cheap, wooden, manmade throne when God the Father has created a beautiful throne surrounded by a rainbow, a symbol of His promise and mercy. He is our King of Kings and Lord of Lords reigning and leading His church victorious to the final days, to the praise and glory of His name.
Read Acts 1 about Jesus spending 40 days before his ascension speaking of things pertaining to the the kingdom of God. And when they all assembled together, they all asked the same question: "Wilt thou at this time restore the KINGDOM TO ISRAEL?" And Jesus's answer was that the time when that will happen is in the Father's hands not His. He did not rebuke them for asking a dumb question. He said it is not for now but for some time later and he gave the apostles a job to do in the meantime.
Read again that third verse you quoted. That is what is happening now or will shortly happen. He is sitting at his right hand until the Father makes his enemies his footstool.
Acts 15:14-17 is perfectly clear. After God visits the nations to take out of them a people for his name [the church], He will return and build the tabernacle of David which is fallen down so that the residue of men might seek after the Lord . . . ".
Replacement Theology is wood, hay and stubble. Why don't you read the prophets. They will tell you, particularly Zechariah.