"Shouldn't have shared his opinion" and "should have shared his opinion in a different way" are two completely different things, and I don't see many people saying the first.
Can someone produce a "diversity culture" critic (for lack of a better term) who provides a good example for Damore? One that is well received across the board?
A big four-paragraph disclaimer at the beginning would've been a big waste of everyone's time, and it could just as easily be interpreted as a sign of hostility or malfeasance. If people want to dislike something, there is an infinity of potential nits to fixate on.
I've always been brash so I've been through the "delivery ringer" many times. The conclusion I've reached is that frequently, the only way to avoid it is to be so opaque and listless in your communication that people aren't sure what you meant.
If you say something people don't like in a non-ambiguous way, they will be mad, and they will insist on finding a reason to dismiss it.
If Damore had pined on the tragedy of the modern economic structure while exhaustively disclaiming every potential discriminatory implication before he began the memo, I guarantee people would've read just as much "intellectual dishonesty", "perverted reasoning", and "loads of bias" as they did now. In fact, they very likely would've read more, taking the content that initially appeared friendly to their POV as a sign that Damore had malicious intent and that he was attempting to hoodwink people by pretending to be "on their side".
As discussed below, if someone wants to dismiss something they don't like, airy, abstract terms like "perverted reasoning" will get bandied around no matter what. These terms are great precisely because their subjective interpretation allows the writer to sound semi-credible in their condemnation without having to specify further.
Damore was fired because once this hit the mainstream press, it was the only way for Google to preserve a strong defense against inevitable discrimination suits.
We absolutely should not construct a credentialist edifice that says only people certified to have gone through brainwa---err, regent-approved programs--- can comment on a topic. That would prevent discussion on most topics, as virtually all topics of interest are complex and have many years of study behind them.
Overall, these comments are still criticizing the how instead of the what, which is what people do when they don't know how to criticize the what but want to express their offense anyway. It's much easier to criticize delivery and in fact it will always happen whenever anyone cares, because delivery is inherently contextual/subjective.
If Damore's paper was rejected from Nature or another peer-reviewed journal, that'd make sense, as it is not a rigorous academic work. It's just a conjecture on the state of diversity hiring and it expresses his reasoning for believing the way he does. If he is so wrong, it should be simple to disprove him, and we can all move on without anyone having to get fired.