So I'm of a long tradition of people who are consistent and living within the same textual tradition. Cool
Then Jesus didn't know what he was talking about. First, why would he have a problem with the fact that humans are sinners? That's built into our nature and the biblical text gives us daily and yearly methods of atonement to deal with that. Next, why would Jesus think that the role of king means "you can never have sinned"? And, of course, there is the fact that Jesus clearly, then, doesn't understand the verse in question. The verse (which is 51:7...in 51:5 he admits that he recognizes and remembers his own sin daily even though he has moved on from it) says that he was born with an evil inclination and through physicality which is what condemns mankind to sin. Sad that Jesus would misunderstand the text -- good thing the Jews are still here to correct his error.
Well, actually, not all men have "died" -- a couple ascended to heaven without what we think of as death. Also, since Adam's time, death has become part of human life (as a function of the shift in nature, not because of a personal sin, so if you bemoan that all men are mortal (apologies to Socrates) then you will have bigger problems with the Jewish messianic idea.
Strange...you accepted a few posts ago that Jesus' father wasn't named David, but that he was from the Davidic ancestry, and yet here, you assume that a singular person named David (which was not Jesus' name IIRC) will rule forever and not die. Also...Jesus died so, um, problem. But let's ignore that for the moment. The text is speaking about that same Davidic ancestry. The line of David will rule forever, not a singular guy named David.
So here you wrap up all your confusion into an goofily elegant ball and wrap it with silly string. The Jewish king is to be a human, same as all the earlier kings. If you want to wade into the argument between Ramban and Rambam about the nature of humanity in the messianic era (whether ALL people will be immortal, including the messiah, or whether all people will be mortal, including the messianic king) you will need a time machine and a lot more learning.