The Limit of Suspension of Belief: Magneto and Xavier
Magneto is a man being torn apart at both ends by Marvel Time. His character is defined by two facets: On the one hand, his having been imprisoned in a concentration camp during WW2; on the other, his status as Xavier's colleague, friend, and rival.
Magneto's ties to WW2 are unbreakable. There hasn't been another tragedy of the same nature perpetuated on such a scale since (and hopefully never will be). Disconnecting Magneto from WW2 would take all the power of his personality from him. And WW2 is becoming more and more distant as Marvel Time slides forward. This isn't an age issue - he was reduced to infancy by Mutant Alpha at one point. Yet another life-extending story could be fit into the increasing time gap as necessary. The problem is that his connection to Charles Xavier is also slipping into the future, because Charles's background is sliding forward.
Originally, as a Soldier in the Korean War, Charles was not so different in age from Magneto. That they could be colleagues made a lot of sense, especially as Charles was involved with psychological treatment of survivors of the holocaust. (One wonders how modern treatments of Charles's past handle Gabrielle Haller - indeed, the lack of any modern treatment on that part of his history is probably because any such treatment is impossible). As Magneto becomes increasingly Charles's elder, a connection of friendship and a true sense of collegiality becomes impossible. They can still be rivals, but the deep connection they developed as young men together cannot happen when Magneto has moved well past young by the time Xavier is even born.
Xavier is generally portrayed as ~40 years of age. If he served in Vietnam (which seems to be the most recent version), he should be more like 60. Of course, Magneto is more like 90. And this discrepancy is only going to increase as time goes on. Its not that Magneto cannot continue to be alive until well after he's 200 if need be - some excuse can always be arranged - its that a 200 year old Magneto relating to a 40 year old Xavier as a friend and equal is an impossibility of character. Even if it would normally beggar belief, Magneto would never condescend to call someone so much younger and inexperienced than him a colleague. The original Xavier really could make a pretense of understanding Magneto's pain, he was close enough in age to understand the horrors of the holocaust firsthand (from working with survivors) and to be treated with respect by Magneto. A much younger Xavier could never hope for Magneto's respect.