The streak breaker pretty much guarantees you get one if you actually play, just not immediately.
They have commented before that Nighthold will pretty much be designed under the assumption that everyone has a legendary, which suggests to the drop rates they had in mind.
Some parts of the game just require active playing, you don't expect them to hand you a trained group of mythic raiders ready to carry you, because its part of the content and you paid for all the content, right?
The content is out there, its up to you to play to unlock it.