Of note: "The origins of characters that were established after I left are not necessarily the origins that we will encounter here" and "It’s not the specific issues I had planned, simply because they seem to have been yanked out of the pile over the course of the last 15 years and actually produced by other people" and "What has also happened is my rethinking of the concept from that point – my looking at the characters, my looking at the school, my looking at the world – has led to some surprising conclusions and revelations about the characters and their world that were not possible to explore when I was writing Uncanny or X-Men, simply because we had to deal with the practical reality of ongoing characters and an ongoing series. By Forever being unique, in that we’re not part of the traditional Marvel Universe, we have a freedom to present their lives and present the characters with challenges that the mainstream books can’t deal with, not the least of which is death."
So, the specific idea of mutants burning out does seem to be an idea that either he had in mind at the time but couldn't do because, essentially, he couldn't put a finite life on characters in an ongoing, or something new that occurred to him since. He has said that some of the plot points are from that period, a mix of stuff he intended to do, things he had the ideas for but couldn't do because they were restricted by the shared universe.
So, this wouldn't have happened. Other plot points in the book would. It's a blend of old ideas and new ideas, seemingly.