I feel like this should be obvious, but food for thought.
Back in BBS, MX took over Terra as a means to extend his lifespan. If KH would make him beyond human, we came to agree that taking over Terra was essentially redundant if MX planned on becoming a higher being (well, it's redundant if KH makes him immortal/extends his mortality, which is a pretty fair assumption). However, I think they fixed this issue in DDD.
One of the rules in time-traveling is that you can only travel to a time where you exist in the flow of time. Perhaps by extending his lifespan, MX has a larger time frame for his counterparts from other times to visit him in (for lack of better wording). In other words, he may have extended his lifespan as a means to exist during the gathering of the 7 lights and 13 darknesses. Without him, there are no darknesses.
Now for the question: Was surviving until the gathering planned, or a coincidence?