Let's try to do it differently. One solution is to only allow truthful statements - this would most likely catch Powell. But that means you have to litigate every defamation claim on its merits, you would need a trial for each to determine whether the statements are "true" or not. Consider the complexity of the Oracle vs Google case, how difficult it is to establish facts in front of a court. Now imagine every giant corp brings defamation lawsuits against its detractors - sue all the random bloggers trying to bright to light that your medicine doesn't work, or that you're using child labor. Of course you have "mountains of evidence" against it, or at least you can claim so, and force a lengthy trial against people who can't afford it. How would you tackle this problem?
Even worse, what if you publish an extensive expose, but some of your facts are wrong? This is actually what happened in the decision that paved the way for the modern defamation defense (https://en.wikipedia.org/wiki/New_York_Times_Co._v._Sullivan) Back then, it was the government trying to silence civil rights activism, nowadays it's more and more large corporations that would rather not have their dirty laundry aired. I genuinely don't see a way to amend the standard without sacrificing a lot of freedom of speech in the process.
What would have happened if Theranos sued all the publications that exposed their nonfunctional product? Would the publications even risk publishing if defamation suits always hang over their heads?