Uhh...
Explain to me the openness of Kavatch. Always a pile of rubble. Every...single...time...no matter what you did.
Kvatch needed to be destroyed to move the story forward the proper tone. With the town being utterly destroyed, it makes the enemy appear to be more dangerous. If you, as one person, could turn the tide by showing up, at that point in the game, with low end equipment, and low end skills (ignore level scaling here) then the enemy would not really be that much to worry about.
Now, could Kvatch have been rebuilt after? Sure.
Heck, I even hated the timer on the final Oblivion gate so much, that I would just go TCL and walk straight to the final spire bypassing all the other ones. I did it the correct way one time, just to see if I could do it, but after that, never again.
Timers on quests are at counter purposed to having an open ended game that you play the way you want to. Otherwise I will have to save before I speak to each and every NPC, in case they give me some quest that I need to drop everything I am doing and get it done now. With untimed quests, I can take quests when I want, do them when I want, finish them when I want and for the most part, do them how I want.
Timed quests make sense in a linear game, as the game's story is what moves you along. In TES games, it is your decisions that move you along the story.