I think the point pretty much was that if you keep involving yourselves with psycho mass murderers, a person with no infrastructure is eventually going to get the power taken away from him. He conquered so many other killers before him (Emilio, Krazy 8, Tuco, Gus, Mike)... who else was left to take him down? There is irony in that he was calling in the Nazi's go help him murder someone, when he ends up getting the wrong guy killed and his fortune stolen, along with literally digging Hank's grave for him. I've got no problem with the lack of buildup for these guys.
Also, I think the writers will tell you it isn't the Aryans that are the ones bringing Walt down. It's his own ego/hubris/incredibly awful decision making.
Edit: Also the Sopranos was about a professional criminal organization. The whole characterization of this show has been about how amateur of an operation Walt has. An organized mob is of course going to have natural, sustainable, long term rivalries. A chemistry teacher turned meth cook who just tries to overrun everyone he comes in contact can only get away with it for so long. He hasn't been in the game long enough to actual develop any actual rivalries.