I'll give it a stab: First, I think you might have been over sold that Walker as a place holder for the establishment meme. Walker won tea party elections, and kind of had one foot in the estab camp but still one foot in the conservative camp. And his anti union actions in Wisconsin were NOT liked by the estabs in Washington.
Which leads me to the next point, which is that a lot of Walker supporters are not part of the establishment at all. And many Walker supporters liked Cruz as their second choice anyway. Thus it makes sense that Walker's withdrawal would help Cruz. Might help Rubio some too. Like I said, one foot in each camp.
Ohhh please God don’t say those words here, Marco Rubio...