Mike responded a few times. He seemed to disagree with ...
Say x is “the probability that a severe bug is discovered in Bitcoin”. Would you agree that a soft fork cannot increase x, whereas a hard fork can?
... on the grounds that soft forks can also create bugs. While soft forks may introduce new bugs in their new specialized features, they (by definition) can't burden users with any existential-threat-bugs that weren't already in the cryptosystem
. Mike has yet to respond.