Well, it depends on how you interpret the quote:
"They will die a death that will last millennia, until all that remains is their code, their history, and in the end, the shell of their armor upon the shell of a man, too easily slain by Jedi." ~ Kreia
I assume she is referring to the death of Jango Fett at the hands of Mace Windu, which would make sense. Jango was a Mandalorian but he didn't have a clan backing him up, he didn't fight in glorious battles across the galaxy like the likes of Mandalore the Ultimate and Mandalore the Indomitable had done. After the Mandalorian Civil War the last of the true Mandalorians were destroyed, and replaced by Death Watch. And then of course the New Mandalorians. After that he was a bounty hunter, working for whoever paid the most. In such a sense we can see him as not fully a Mandalorian, he just wore their armor. Was he a 'shell of a man'? I would say so, the Civil War left him scarred and distanced from the Mandalorian traditions.
And the last line makes sense too, he didn't even land a single shot on Mace Windu, he was decapitated after five seconds. But was this the death of the Mandalorians? Was Jango the 'prophesied' Mandalorian? Well seeing as their have been other Mandalorians after him, probably not. But then again, I don't know much about the guys after Jango - to me the just sound like glorified bounty hunters.
One things for sure, the Mandalorians will never be like they used to me. The age of Mandalorian clans uniting together to glorious war against the galaxy are over.
EDIT: We also have to consider what makes a 'True Mandalorian'. Jango Fett was taught in the Mandalorian ways from a young age. Boba Fett was not. He was bounty hunter from a young age and only adopted the Mandalorian ways as a promise the previous Mandalore. So I would assume he had little grasp of what it meant to be Mandalorian. I don't know about the other guys, but I doubt they were ever trained as Mandalorians.
Mandalorians not rallying together to fight the Galaxy again? Three Words: Star Wars: Legacy
Boba Fett was raised like a Mandalorian child. What do you think Jango was doing all those years when he was raising Boba from birth. He was grooming him to be his successor. He most likely brought him up the same way Jaster had did him until his death.