When Star Trek: The Original Series was brought back as Star Trek: The Animated Series, there were a lot of changes. Walter Koenig was left out of the series, there was a different alien on the bridge besides just Mr. Spock, and the animation was mediocre at best, to name a few. Those changes didn't stop the series from delivering some pretty stand-out episodes like "Yesteryear" when Spock has to go back in time to save his younger self.
The Animated Series aired for twenty-two episodes between September 8, 1973 to October 12, 1974, with those counting as two seasons. And though the ratings were high, the show didn't reach its targeted demographic.
Gene Roddenberry's son, Rod Roddenberry, wasn't too impressed at the idea of the series initially and, according to a quote recorded in The Fifty Year Mission The First 25 Years by Edward Gross and Mark A. Altman, he "dismissed it as nonsense without ever seeing it."
His opinion changed after watching the series, and he admitted that he thinks of The Animated Series as "the fourth and fifth year of the voyage" as "the caliber of the stories was on par or even better than a number of The Original Series episodes."
Though there were many good episodes, it was hard to draw in the kids who were looking for a different kind of Saturday morning cartoon. Adults, on the other hand, were tuning in, and there's no doubt they would have continued to do so. Just look at the popularity of Star Trek: Lower Decks and Star Trek: Prodigy, and those have better animation quality.
Now, fifty years past the cancellation, The Animated Series is considered to be one of the best of the franchise, and viewers still tune in on Paramount+ today to watch the fourth and fifth year of the Enterprise's journey.