There's a downside to loosening up the mental resistance to mind-changing - you're more susceptible to cult indoctrination.
You can look no further than the Rationalist community who have internalized this to such a degree that cults are endemic to the community. Sure, there's positives to being open to changing one's beliefs, but like all advice, it's contextual. Some people probably do need to loosen up, but they are the least likely to do so. Those who hold their beliefs too loosely, could stand to tighten that knot a little more.
You can look no further than the Rationalist community who have internalized this to such a degree that cults are endemic to the community. Sure, there's positives to being open to changing one's beliefs, but like all advice, it's contextual. Some people probably do need to loosen up, but they are the least likely to do so. Those who hold their beliefs too loosely, could stand to tighten that knot a little more.