Admittedly I am largely ignorant of the Star Wars Legacy period, so your going to have to expand on that. Although my basic knowledge of the period tells me that there were no major groups that waged war against the galaxy like the Mandalorians of old did. Correct me if I'm wrong. In a sense you could say the Mandalorians died with Jango. After that they were just scattered mercenaries if that, they were only 'revived' many years late by Boba Fett. So maybe Traya foresaw the death of the Mandalorians but not their revival.
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.
Oh and concerning Boba, there is no evidence to suggest that he was trained by his father in the Mandalorian ways. He was taught in combat etc, but not in how to be a Mandalorian. We can assume that Jango died before imparting that knowledge.
Question: Were any of the 'Mandalorians' that followed after Jango trained in the Mandalorian ways from a young age by a recognized Mandalorian?
But yeah, I reckon the Mandalorians may have some kick still left in them. However I believe the death of Jango marked the beginning of the end...