Roddenberry’s change of World War III from something in the future of the 60’s to something in the future of the 80’s strikes me as the first manifestation of the trend among some creators and fans to constantly be wanting Star Trek to be a truely possible future from ‘now’. Rather than the default in my mind, which is that Star Trek is a fictional universe that needs not correspond to our idea of the future as we might think of it today, at the expense of continuity with what’s already been established in the Trek universe.
Well, the approach that was taken with TNG was the typical approach for most fictional continuity at the time. Modern audiences have become attached to the idea of continuity as a rigid and immutable thing, but that's partly because we have much easier access to the work as a whole through home video, Internet references, etc. In the past, that holistic view of an overall continuity was harder to get, so creators tended to be more flexible about changing the detailed facts and just pretending it was still consistent.
For instance, before he did the Trek movies, Harve Bennett produced
The Six Million Dollar Man as a weekly series, though he hadn't been involved with the three pilot movies. When it did episodes revisiting events from the first pilot movie, it kept only the broad strokes and freely retconned their specifics to fit the story (even beyond the retcons made in the latter two movies, which replaced Darren McGavin's Oliver Spencer with Richard Anderson's Oscar Goldman and changed Steve Austin from a civilian astronaut to an Air Force colonel). One episode that revisited the events of Steve's bionic surgery and recovery from his accident replaced the nurse who'd been his love interest in the pilot movie (played by Barbara Anderson) with a different character played by a different actress. The details didn't matter, because audiences' experience of a TV series was not as encyclopedic as it often is today. What mattered was what the
current story needed.
Roddenberry was from that same generation of TV producers, and thus he was perfectly willing to rewrite the facts of his series' continuity. He didn't see TNG as an exact, slavish continuation of TOS continuity; he saw it as a soft reboot, a chance to reinvent
Star Trek to fit his modern view of it and correct past mistakes and discard the parts from earlier productions that he wasn't happy with. The people who create stories don't see them as fixed, permanent things, but as the end result of a process of trial and error and experimentation and change. So naturally they're willing to change their creations still further given the chance. The story doesn't control them, they control the story.
So
of course the Gene Roddenberry who was making a Trek TV show in 1987 was not going to keep the bit of TOS continuity that put a global war in 1993, because that would've been stupid. He wasn't making a slavish tribute to decades-old continuity, he was making a TV show for 1980s audiences and future audiences. Writers are like anyone else -- they learn from experience and try to improve and change and make up for past mistakes. That's why it makes no sense to demand that a writer's creation remain absolutely fixed and shackled by its earliest ideas. That denies them the right to improve their work over time. Continuity should not be seen as the exclusive, overriding goal of fiction. The goal of fiction is
creativity, and that requires the freedom to innovate.