I have this vague feeling I’ve heard this fallacy named before but my admittedly relatively perfunctory search of various lists of common fallacies hasn’t turned it up. Maybe I’m wrong and it has no name.
The fallacy is consideration only of first order effect of a proposed change, without considering the changes that will result from the proposed change. It can occur in any context but I see it most commonly concerning human behaviour: the proponent or detractor of a change argues for (or against) the change without considering changes in behaviour that will result from the change, which may negate benefits (or avoid detriments) that might otherwise result from the change.
So for instance:
“If the government is entitled to a backdoor into that encrypted messaging system they can listen in on the plans of terrorists” - except that if the government is entitled to that backdoor, terrorists won’t use that messaging system.
“If this street is reduced to one lane there will be a huge traffic jam from all the cars that now use it” - except that if that street is reduced to one lane, people will (if they can) drive a different way.
And so on.