Note that they were saying ban development of. Not do away with. And not necessarily for the rest of time. There are obviously loads of properties that fit the bill and would go undeveloped for many years.

That said, I do think banning them (even just by developers) is unnecessary if you provide zoning incentives instead. If you give developers the option to split up land because it will give them a higher return, they're typically quite quick to do that, at least in Australia. That wouldn't prevent owners or speculators from building their dream standalone house.