Actually in the campaign you more reasons to keep playing till the end, the plot, you wanna see how it will go, who and what will say, and revenge, since the enemy will start with an advantage most of the time, "remember when your dragon almost wiped me out in the beginning, because I do, EAT GIANTS! GIANTS EVERYWHERE!".
As for no end, I'd have to disagree:
Firstly, there are plenty of games that don't have an end, getting a space victory in CIV and quitting the game in AoE are the same, except you get a cutscene in CIV.
Secondly, in AoE 3 the AI will figure out that he can't fight you in the open and he'll fortify at the capital, it won't save them when you have map control but it usually tends to set the scene for a battle of epic scale, even more then every other battle until then, one last great battle to claim victory as opposed to the last boring breaths of other 4x where you are just waiting for the arbitrary turns before you get the victory cut scenes.
So cutting 2 points off for not having a boring, wait for a timer that cannot be stopped, ending is just hypocritical, especially for a game where tactics matter just as much strategy, where you can get overconfident and have your main army destroyed and your cities open to attack in what was supposed to be a mop up operation. Sure it is going to be more boring then the early struggles for map dominance, but you can still be surprised, which is not something you can say for most of the other 4x games, where map dominance cannot be lost.