@LostInParadise, your most recent comment makes it seem like Christianity’s theology could just be written (or rewritten) from scratch. I don’t think this is the case, since Jesus almost certainly existed and almost certainly really did get crucified at a young age.
Allow me to take this opportunity to address a common atheist argument that pops up a lot—the idea that Jesus never really existed and was an invention from thin air by Paul and Peter. In my view, this violates Occam’s razor—it creates more holes than it explains.
First of all, the idea that there was a Jewish cult leader named Joshua around 30 A.D. who got crucified by the Roman government is not an extraordinary claim. We know for a fact that there were many such cult leaders in late-antique Rome and Judea. Yes, Jesus did not leave any physical evidence that “proves” his existence. But neither did lots of historical figures, including Siddharta Guatema, Johnny Appleseed, and Sargon of Akkad. (All of these historical figures, like Jesus, have associated legends surrounding them, but surely you wouldn’t say they are all imaginary figments?) Extraordinary claims require extraordinary evidence—for example, Sargon or Jesus being the sons of various deities—but ordinary claims can easily enough be inferred, especially if they fit into the historical record we have.
Secondly, if Jesus did not ever exist—if Paul and Peter invented him—then where did his cult come from? Paul started writing his letters in the 50’s A.D. By that time, there was already a well-formed cult centered on the figure of Jesus. In fact, this cult had already splintered into sects! (See, for example, Paul’s letter to Galatians, where he berates a rival sect of Christians for promoting circumcision). It seems far simpler to believe that Paul and Peter grafted themselves onto an existing cult than it does to say that they invented this cult out of thin air, when it seems to have been quite developed by the time they started.
Thirdly, it is actually quite rare for cult leaders to make up historical figures out of thin air. Far more common, cult leaders die and then later cult leaders co-opt their followers—often elevating or deifying the original cult leader in the process. For example, Jesus’ cult seems to have done just this with John the Baptist’s cult. This happens in modern times as well—look at what Stalin did with Lenin. Lenin became something like a god, who conveniently supported Stalin’s interpretation of the “scriptures.” Reading Paul’s letters, it seems patently obvious that he is an opportunistic demagogue who saw this cult centered around the now-dead Jesus as a ripe opportunity.
So, the long and the short of it, I don’t think it makes sense to speculate about how Christianity would “make more sense” if its theology made Jesus survive into old age. Christianity seems to have formed around Jesus’ untimely death.