All of the series that came after Star Trek: The Original Series until Star Trek: Discovery ran for seven seasons. It started with Star Trek: The Next Generation, but there was no guarantee it was going to happen for future series as evidenced by an interview with Ron Moore in the August/September 1997 edition of Star Trek Communicator, The Magazine of the Official Star Trek Fan Club.
Moore told the magazine that the sixth season of Star Trek: Deep Space Nine might be its last. It wasn't that Paramount didn't want another season; it did, but the actors were on six year contracts. The producers just assumed they would only run the show for six seasons, but even then Moore said "we won't be tying it up clean and simple."
Season five of Deep Space Nine had the crew embroiled in the Dominion War, and while it would have been difficult to wrap things up in one more season, there would have been some benefits. Most likely, Jadzia Dax [Terry Farrell] would not have been killed, and Worf [Michael Dorn] could have finally gotten a happily ever after. The same could be said of Odo [Rene Auberjonois] and Major Kira [Nana Visitor]. And Captain Sisko could have possibly remained with his family instead of going with the Prophets.
While there were some standout episodes in season seven like It's Only a Paper Moon, season six excelled with more good episodes than bad. Sons and Daughters, You Are Cordially Invited, Favor the Bold, Far Beyond the Stars, In the Pale Moonlight, and The Sound of Her Voice are all exceptional and easily rewatchable.
So while Paramount got its seven seasons, Deep Space Nine could have easily wrapped with the sixth being its final one, and the series would have still gone out on top. Some fans think it might have been better that way.