this is the very reason that IF organizers were to decide upon such a change, they should implement it out a year such that it has no effect upon anyone's already made plans
Apply the change only to those seeking entry for the first time. the purpose of entry qualifications has never been to reduce the field allowed to sign up, but rather, to limit the sign up to runners who actually have the fitness to participate safely. Narrowing the qualifying window for new participants arguably fits that purpose, whereas narrowing the window for veterans does not. It would be saying to veterans, "if you don't want to run this event at least every other year, we don't want you."
And in response to a comment, surely you have run a marathon in the last two years, haven't you? No. I am 68 years old, for example, and I have run for 55 years. No, I don't run road marathons any more, in fact, almost never run on roads, I run very few organized events, but I run up fourteeners regularly, and I do know how to prepare to run up Pikes ;-)
Being an older (and slower) runner these days, changes my perspective. Here is another thing organizers might do which might help organizers deal with their problems, while assisting older runners in dealing with ours ...
Allow older runners of some specified age to start earlier, then face the same cut-off times. This can allow older runners to joyfully stay at it with less risk of failing to meet the cut-off, and being less likely to burden rescuers and aid workers.
For example ... what if runners aged 65 and older were allowed to start an hour earlier than Wave 2, half hour earlier than Wave 1...
* this group of runners is now not at the traditional Wave two start; maybe you can allow additional runners into the race. If not, wave two is less crowded.
* Older runners can have an extra hour before the cut-off takes them off the course.
* Older runners are both safer and more likely to finish
* the older runners being only a few, will be well spread out before encountered by the field
* younger runners will get to SEE the older runners
* rescuers and aid stations will be fewer turned around runners since they would have an extra hour to make the cut-offs.