Check these texts please. In Isaiah 9:1-7 “The people who sat in darkness saw a great light…. And of His Kingdom and Peace there will be no end.” In Math.3 John announces this kingdom and in Math.11:13 Jesus confirms his announcement so It seems to me we have pretty good evidence as to when this eternal kingdom started. I have a hard time correlating the arrival of the New Jerusalem to AD70 but agree that Math. 16:27 does present a challenge and the kingdom Jesus discusses with His desciples on the way down the mountain from the transfiguration in Math.17 is certainly that same kingdom. If Jesus meant to indicate visualizing the kingdom as in a realization that the kingdom had arrived then it would seem this was realized in Math.17:11-12. If He meant to actually envision the Kingdom then John certainly envisioned it on the isle of Patmos. Either way thr prophecy was fulfilled, some present saw the Son of Man coming in His Kingdom.