Wishing and hoping for people to be better than they are isn’t a solution. Just because traffic calming is more expensive, that’s not a reason to not do it. It is something that needs to be done if you want to break car dependency.
We should be doing that, but local councils don’t have the money after more than a decade of tory austerity. I also believe that driver’s should be able to drive below the speed limit even if the road isn’t correct for it, because there will always be places like that (around construction, for example), and like you say we can’t just wish and hope for them to follow that rule so some enforcement is needed.