There is an irony in that the people who claim that 20 mph limits can't be enforced, are usually trying to oppose them. Perhaps they don't like the idea of observing the law themselves and then seeing others break it. Who knows? But the lack of will to enforce a law, is no reason not to have the law.
Enforcement is not necessary to make a difference.
Even if 20mph speed limits are not enforced, they will still make a difference to the vast majority of drivers who drive at speeds that respect other road users and who drive within the law. By simply putting signs up denoting that the speed limit is 20mph, it clearly signals to drivers that 20mph is the maximum speed which the community and the law expects from them. That will cause most drivers to maintain a reduced upper speed.
How 20mph speed limits would be enforced.
20mph speed limits would be enforced in exactly the same way as other speed limits are. Either a speed limit is enforced or it is not. Either the police will engage in enforcement or not. Therefore there would be little difference to the current situation regarding policing and enforcement in 30mph limits.
The whole 'It can't/won't be enforced' argument is based on the premise that people only observe the law if there is a punishment if they are caught breaking it. Surely the purpose of law is to give people clearly defined parameters of acceptable behaviour to which society expects them adhere to.
In terms of speed limits, they are there for peoples own safety as well as for the safety of others, so every driver has at least some incentive to observe the law with regards to speed. If they get caught breaking the law, then that could lead to punishment under the law, and (most) people do not want that. Therefore, the law does not necessarily need to be enforced to make a difference, but it does need to exist.