I think that demagogues such as Trump and Johnson can get away with such political shape-shifting for the same reason that they can get away with such frequent and quite blatant lying. Of course politicians have always lied, but in the past they've always tried to cover it up ("I did not have sexual relations with that woman"), but the new populist politican just doesn't care that everyone knows they're lying, and it doesn't weaken them in the way it did with, say, Clinton, it actually strengthens them as it helps to build up their carefully-crafted persona as the embattled outspoken outsider railing against the corrupt establishment. And this is all possible because the populists don't appeal to reason, using logical arguments, they instead appeal directly to emotion and the carefully curated prejudices that have been instilled in large sections of the populace. The opposition don't seem to have understood yet that when for example Trump has a rant about Mexicans, it's impossible to counter that by saying "well, actually Mexican immigrants contribute x to the economy, provide vital labour force in y industry, etc". Same reason that when I use facts and logic in a discussion with BADecker, it has no effect.