Yes, TOS/DS9/VOY all had fairly final endings... at least where the recurring characters were concerned. But life on DS9 goes on with Colonel Kira Nerys in command after Sisko ascended to the Celestial Temple. Life on board the Enterprise continues with the next captain after Picard retired. But enemies of the Federation remain, namely the Dominion and the Borg. I think there could be room for a movie about it, with a whole new set of characters. But I have to admit that the odds are very remote.
They leave it open-ended like that because:
1. some fans don't want a final, final end, where characters are killed off, retired, or what have you. Not every aspect needs to be fleshed out, and not every avenue of every possible story needs to be explored. Some people like to see the open-ended finale where the heroes just sail off into the proverbial sunset, and not everything has to have an "endgame." *
2. It leaves room for authors to write books. Everything you are talking about is there.
* Speaking of which, the Borg did have an "endgame," in Endgame... Their trans warp conduits were destroyed and a virus was introduced by Janeway, killing the Queen, and probably accomplishing what Picard should have done in I, Borg. That was the borgs' last chronological appearance on screen. Between their defeat, 7 of 9 advising the Federation on Borg Tactics, and the fact the Borg would likely splinter, scatter,and some would even self-liberate, after the fall of the collective as what is explained in Descent, and in various Voyager episodes where this also happened, the Borg are arguably done, or at least a threat that is greatly diminished.
The Dominion was defeated, namely the founders, also by a virus. The Jem Hadar fleet, despite being powerful, was not strong enough to stand against The combined fleets of the Federation, Klingons, Romulans, Cardassians. Why do you think they allied with the Breen, anyway? With the defeat of their gods, I would not put it past the Vorta and Jem Hadar to commit mass suicide, too. Once the Founders surrendered, the Breen would have abandoned the Dominion, and went back to their xenophobic ways, as well. If you recall, Odo went back to the founders to ensure they would not invade the Alpha quadrant again, and to teach them a different way than conquering civilizations. So they had an "endgame," too.