"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."
Nothing in Star Wars pretty much makes sense, but the basic idea of the Mandos in the Clone Wars era being remnants of a borderline dead culture and Jango Fett patterning the clones on them as a last, desperate attempt at preserving something wasn't a bad idea. A scattering of Mandos being forced to be hired guns because they're a failed warrior society isn't impossible to maintain even in the Empire era. God knows what happened to them under Disney Wars, can't even be bothered to check. As usual, the more you explain....