Probably not stupid
Your difficult boss, customer, prospect, voter, student... probably not stupid, probably just uninformed. There's a huge difference.
Every person makes decisions based on their worldview and the data at hand. If two people have the same worldview and the same data, they'll make the same decision, every time (unless they're stupid.)
So, there are plenty of times where a lack of information leads to a bad decision. Plenty of times where an out of sync worldview leads to an out of sync decision.
When the board of directors embraces a fading old media model instead of embracing a strategy that leads to rapid growth, it's probably because each of them started with a worldview about the way things worked and were going to work. Add to that little direct experience, and it's no wonder they decided what they did. You would too if you were given the same resources to begin with.
Changing worldviews is very difficult and requires quite a bit of will. Changing the data at hand is a lot easier, and that's where marketing can really help. If you, as a marketer, can package data in a way that people with a certain worldview can accept, you move the conversation forward far more quickly than if you merely dismiss the non-customers or the doubters as stupid.
In my experience, a closed-minded worldview ("I can't read that book, I disagree with it") is the most difficult hurdle to overcome. But a closed-minded worldview doesn't mean you're stupid, it means that you are selling yourself and your colleagues and your community short.
The easiest way to grow is to sell to people who share a worldview that endorses your position. The most effective way to grow bigger than that is to inform those that disagree with your position--more data in a palatable form. And, unfortunately, it turns out that the best way to change the world is to open the closed-minded.