Not every car will be perfect, you will never get rid of all mechanical failures or flat tyres. Once you've accepted that will happen, you have to deal with it, not just take away the hard shoulder and hope no one gets killed when motorway traffic runs into the back of the broken down vehicle.
If the smart motorways had been built as promised, and the systems actually worked, you might have a chance of arguing they are of an overall benefit, but when cameras are out, software crashes, refuges are a couple of miles apart, etc there can be no argument that smart motorways are killing people who would otherwise be alive.