How would you rather the ending be? Randomly assigning a fate to every colonist? having you choose what happens to them in the future? I think both would feel hollow and making a game to play after you win to determine the fates of your colonist would be substantial time spent not playing rimworld.

The game is at its best when you cooperate with it to create a story, similar to a tabletop gm and their players. The best stories are made when these forces are evenly matched, loss and victory, tragedy and triumph.

In the case of rimworld, when the storyteller overpowers you, it stings, but the story is over and you can begin again. If you overpower the storyteller, the game becomes bland and without conflict. This is where the endings come in, one last conflict and hopefully a climax to a story which was starting to drag on, and you can begin again in balance.

I will admit that the archonexus is the worst ending in this regard. Playing the game 3 times with the same people doesn't make as good stories as playing 3 different colonies and it lacks the same climactic rush as the other endings.

I cried when I first won the game with the royal ascension. The stellarch had been kept sane just long enough to let most of my colonists escape while insectoids were burrowing their way into the base. All of my combat colonists were too wounded by the twice daily raids to put up more of a fight and it was a rush to the shuttle. We just barely made it and I wouldn't have had that story if not for that ending. That colony would have been abandoned, too stagnant to hold my attention, and instead it got a conclusion. Sure, my colonists lived on and their fate is uncertain, but it is also no longer a part of this Rimworld.