There is no excuse for bad writing, they could have been more creative. Killing Ulfric thus the Stormcloaks go into disarray, sparking an insurrection within a rebellion would have made it all the more interesting.
I think they ran out of time really. There's just so much content in the game that seems rushed. Basically the whole College of Winterhold, for example.
Also, I'm still amused as to why people think Alduin "happened to show up at the right time". If he knew the PC was the Dragonborn, he found him/her a curious challenge. Dragons are like Nords like that. 

 If he did not, as I assume, he couldn't know he accidentally saved his worst fears. I repeat that Dragonborn might've even become Dragonborn just then. Akatosh could've arranged the whole event and its ridiculous timing.