I am not sure that even you can say that about the last 100 years. When David O McKay died in January 1970, there seemed to be some debate among the apostles about whether Joseph Fielding Smith should become the new church president given how old he was (his early 90s). It would seem that Harold B Lee, who was next in seniority after Smith and at least 20 years younger, advocated for the practice that had been followed up to that point of the president of the Twelve becoming president, and that view won the day. You can find discussion of this in Goates' biography of Harold B Lee. I think it may also be discussed in Gibbons' biography of Joseph Fielding Smith. The accounts are not 100% forthright about the apparent controversy, but given how far they do go in talking about it, I think it is probably safe to conclude there was a real disagreement about what should be done.
I seem to recall when Hugh B Brown was a counselor in the presidency he proposed creating an emeritus status for apostles past a certain age. He would have qualified for emeritus at the time. I guess he never sold his idea to the other brethren.
I also heard an apostle (who?) answer a succession question that the man next in line may not succeed to the presidency if he didn't want it. (Likely never to happen).
Thanks to the marvelous Church programs that keep me busy from early morning to late night seven days a week, especially on the Day of Rest, I have no spare time whatsoever to think about all this Succession stuff.
By the way I probably won't be responding to any responses here this morning because I'm watching an awesome fireside talk on BYU tv. One of the Brethren is speaking about the importance of Obedience to the leaders. Wow that is such an amazingly awesome topic :)
I did a search for talks on "Honesty about Church History" but couldn't seem to find any as it appears that the Spirit has inspired the search engine instead to point me to a talk about Obedience to the Brethren. So I guess that these will be the types of talks I'll be watching for the rest of my life to help me be better correlated to be what the Church would call open-minded.