Moreover, I don't buy the notion of this "holy" voting base in IA. Iowa has routinely turned out jerk pols, like Grassley and Harkin. But even if Trump were to "lose" IA (impossible, given that it's a proportional caucus state) he'd still win delegates that Cruz can't get elsewhere.
Cruz isn't even on the radar in NC or GA or FL or NH. He's second or third, but distantly so I think, in SC and NV. In short, no. The "invincibility" is there because it's there, not because it's perceived to be there.
FAct is Trump reacted like a scalded dog when 1 poll showed him behind.
Neither you nor I know what happens to all these polls IF Trump loses in Iowa. All your assumptions rest on polls which can change a lot. Cruz has the organization and $$$$ to be there if Trump falters. Huck and Santorum didn’t have the org & $$$to capitalize on their Iowa victories, Cruz does.
Trump knows this hence the ballistic reaction when Cruz went ahead in 1 measly poll. The debate will give Cruz more airtime this week due to his rise in the polls and the media perception of an opportunity for a cat fight between Trump and Cruz.