Almost as dubious as your reasoning. The amendment said that he could not hold any office, not just elective office. That would include cabinet and senate, as well as House and state and local offices. Whether or not he ever planned to run for office again was irrelevant. He was prevented from holding almost any office whatsoever because of his actions in leading the rebellion against the United States. He had been punsihed for those activities. Call it what you will, Chase's reasoning concerning the 5th Amendment was sound and it saved the U.S. from a divisive trial and conviction of Davis. In the end perhaps it was an excuse to drop the whole matter and get on with the task of putting the southern rebellion behind us, but to call it invalid or bogus is wrong.
I do think that was part of the intent. But if you were unjustly accused of a crime, how far would you go to clear your name? To me, whatever it took. Davis was in jail, he wasn't in any position to hold any office. Until the trial was over and Davis released to actually hold office the 14th did not punish him. And a trial to prove his innocence is what Davis wanted. And a proof of innocence was the one thing the US could not allow.
Chase's reasoning did NOT, in fact, save anyone from anything. Johnson's pardon settled the issue before the Supreme Court took up the issue of Chase's reasoning from the divided court below.