Art takes a bit longer (though with the way things are going that excuse is starting to wear a bit thin).
Eh. Berserk has been running since 1990, and it's entirely possible that it'll keep running for another ten years at the rate it's going. I'm prepared to deal with that. Miura doesn't keep Berserk running at such a slow pace because he's lazy - on the contrary, it's the only work he does (playing Idolmaster doesn't count)
- he does it for quality control. As a result, the art and writing of Berserk have been well above average for twenty years.
If it takes Sadamoto ten years to come up with an ending to the manga that doesn't suck, then I'm prepared to deal with that too, all the more so because unlike Miura, Sadamoto actually has other things to do. It's worth noting, too, that Sadamoto's style and subject matter is substantially more detailed than the shovelware manga that make up the Big Three and their ilk. Drawing Evas is hard.