By this logic, governments shouldn't legislate anything or have any kind of policy. Child benefit? Scrap it in case King Herod takes over and has an ready made hit list.

This works both ways though, ie there’s no point opposing the laws on the grounds that they might be abused in future because the future sovereign parliament could just pass the same abusable laws.

It's not always easy. If they don't have large majority, some with a bit of conscience might go against the party.