This is a simplistic take. Of course trust is a pre-requisite to a healthy team, and people should feel comfortable speaking out, there's no debate about that. However, transparency is not a silver bullet. I've seen plenty of teams that trust each other but don't get very much done. And if what you are doing is large and involves more than a two-pizza team or challenging tradeoffs then effective communication becomes much harder. If everyone just speaks their unfiltered thoughts without thinking about how those words will be perceived there is a high likelihood of confusion and churn. There is also the matter of expertise, and the fact that many important truths may not be grokked by all stakeholders, so just blurting them out may lead to furrowed brows and unproductive lines of questioning—or worse—a bad decision fueled by misunderstanding.