I agree with the premise that the final state is not inevitable, but I fail to see how the GM owes the players any particular type of resolution. That depends entirely on what the group desires. If the players all want a gritty combat simulation from start to finish, then, yeah, you should deliver or let someone else step into the GM role. But that's hardly universal. My current players are big fans of narrating through the tail end of long combats, despite using an intricate second-by-second tactical combat system. Once the adrenaline has started wearing off, we're ready for the next element of the story. Finding out whether Gregor the Buttkicker has 12 or 8 hp doesn't add enough value to the story to be worth the minutes invested.Even if the over-all outcome is clear, the final state is not inevitable. You may know that the PCs are going to win, but a win where they slay their opponents is a far different win than one where somebody gets away, or a win where one of the PCs is severely wounded; and you owe it to the players to play that out fairly.
Again, I am not saying that this is the One True Way. But I will say that it is a perfectly valid and fair way to handle it. As time has become more precious, my players and I increasingly prefer this.