I hated the death scene. The way it was shot made it seem as if Jango landed where he did, and said "Here I am Mace baby. Lop off me noggin'."
Then again, Mandaloran armor is resistant to lightsabers. Weakspots need to be targeted, most notably, the neck area. So, perhaps it was correct for the Mandaloran idiom.
You may be onto something re: the finality. I think it certainly was strong content for the movies, but they were a darker offering in some ways than Episode IV - VI. I think the main reason it was done, though, was to show Mace's superior skills in a quick, non-lecturing way.
Remember: we saw Jango mix it up with a variety of campers, most notably Master Obi Wan. They really went to town, and you saw Obi Wan (a skilled master) struggle and ultimately, lose to Jango's superior skills. So, when Mace comes along and does a quick slice and dice, it's a very potent message that says without too many words: This guy really is the baddest Jedi Master in Dodge City.