The biggest risk of "existential risk mitigation" is that it will be used by the "precautionary principle" zealots to shut down scientific research. There is some evidence that it has been attempted already, see the fear-mongering associated with the startup of the new collider at CERN.

A slowdown, much less an actual halt, in new science is the one thing I am certain will increase future risks, since it will undercut our ability to deal with any disasters that actually do occur.

One point Hayek made in his last book, The Fatal Conceit, is that you need to be aware of how others will use whatever policies you support. His arguments were primarily about how "well meaning" academics ended up paving the way for the Communist dictatorships, but the general argument is equally true of many other policies.