This is a horrible idea.
The Panglossianism that will result from this is indicative of the bubble hype which will destroy Silicon Valley, Y-Combinator and Hacker News.
In fact, this is the worst idea I ever heard in my life.
The guidelines aren't asking you to behave like Pangloss and think everything is perfect. It asks if you do have criticism then address it respectfully and constructively.
According to what moral framework are you precisely defining "respectfully" and "constructively"?
There are plenty of examples in the world where bringing up logical, concise, and polite criticism invokes the wrath of a cabal of power-holders and social media demagogues, especially in a world that promotes the postmodern idea of "interpretation, not intent, is reality"
If it helps you can take respectfully to mean "the recipient will not feel angry or attacked due to the message tone" and constructively to mean "the recipient will finish reading the message feeling positive with ways to improve their work or actions".
So continues the descent of HN...