Dotcom was at worst a white collar criminal. He wasn't a drug trafficker or anything of that nature. There was no reason to expect violent resistance to an arrest. Two or three uniformed officers would have been enough to make him go quietly.
Destroying evidence is a crime, so they can always just arrest people who do that for that.
How is this different from a corporate email retention policy that auto-deletes emails older than 6 months?
I mean, sure, you can be required to override the policy under certain circumstances (e.g. when notified of an upcoming lawsuit), but what if the only person capable of doing that is being held under arrest? (Or just cannot be contacted)
My intuition is that, in the United States, it would be a 5th amendment violation to require an arrestee proactively disclose the existence of such a kill switch.