Pitchwife,
the main problem with my scenario is that I am not Tolkien
Thank you for interesting response; of course I was just kidding.
I really like this idea of symmetry in two powerful men lifespan. I would just add that Theoden (when he was sane) was completely attached to his people, whereas Denethor is clearly remote. I can suppose that for Tolkien this could have been a borderline between a proper monarchy and tyranny.
However, I'd like to draw your attention to two forces that could, in my opinion have influenced Denethor's view on things in a positive way had he survived the siege. Firstly, it is his fatherly feelings. Secondly it's his perpetual bid for honour. I wouldn't say he was an ordinary power freak; what was his fetish was rather matter of STATUS. But he wouldn't have admitted this, so he devoted himself to guarding his HONOUR instead. The fact that during the siege he was not helpful at all would have been a great shame and if a bit of common sense had remined in Denethor he would have realised that resigning is the only way to save some honour in this situation. This thought would either have driven him completely mad or would have made him to start coping with reality. At the same time the return of the king could have appear a much better formal reason to resign than just to leave power to the son, for it shifts the attention from previous failures to current needs. I can imagine Denethor half-heartedly agreeing to hand the power over "in order to avoid another kin strifee", following the common wish for Aragorn to rule Gondor. Then he could either have made another attempt to die for Gondor in some more sensible way or have enjoyed the status of a father of a gondorian Prince.
PS. I can actually find some correspondence with Theoden's destiny even in my "scenario", where Denethor also dies fulfilling his errand, but unlike Theoden who dies with his people, Denehor does it in solitude, making his fight his own way.