It always has been. TOS started out as a show about an Earth ship; the Federation was a retcon midway through season 1. TNG portrayed a Federation that had been at peace for generations, but then in season 4 they retconned in a Cardassian war that had only ended the previous year. Anyone who thinks that Star Trek has ever had a perfectly consistent continuity has not been paying attention.
And as I mentioned before, Gene Roddenberry already retconned WWIII (aka the Eugenics Wars) in TNG: "Encounter at Farpoint," when he bumped it from the 1990s (then just a few years away) to the mid-21st century. When he made TNG, he considered TOS to have happened only in broad strokes and intended to rewrite and update its continuity for a modern audience. It was later producers who brought it back into greater consistency with TOS. Roddenberry wanted Trek to be a plausible vision of the future, and to him, that meant updating it for its current audience.
Heck, Trek chronology was retconned a lot of times in its first couple of decades. "Tomorrow is Yesterday" and "Space Seed" put the show 200 years in the future, while "The Squire of Gothos" put it 900 years after Napoleon, which would be about 700 years in the future. "Metamorphosis" put it 237 years after Zefram Cochrane's birth, which probably put it more than 200 years ahead. TMP put it more than 300 years after Voyager 6 was lost, and since only 2 Voyager probes had been launched by 1979, that implies something after the 2280s, possibly quite a while after, since it would've had to travel pretty far to hit a black hole; but TMP is now assumed to have taken place around 2273. TNG initially said Data was "Class of '78," but then gave the calendar year as 2364, and it's only since then that the timeline's been treated more or less consistently.