I'm afraid you are simply never going to stop aircraft crashing at air shows. It has happened before and will always continue to happen. I agree you need to look at the risks, and whilst I'm firmly of the opinion that the T7 crashed at Shoreham as a direct result of pilot error, you cannot and should not try to reduce the risk to others (non-aircrew) to zero.
It was right that after Ramstein the 'on crowd vector' was looked at. The Frecce's 3-way pass accident was possibly waiting to happen, and the outcome was always going to be serious, but you simply cannot sterilise the entire area under which a displaying aircraft could crash. Even coastal venues have an element of risk to the crowd. Whilst the JP at Southport dishing out of its barrel roll didn't crash, it could well have done and would probably have done so on land.
The appropriate display line (100, 230 or 450m from the designated crowd line) does not extend much beyond the venue's boundary. I can't see how you can police exactly what happens below it never mind the possible fall-out area that an aircraft -or part of - could reach.
Life is full of some risk. Didn't some jogger years ago get hit on the head (and killed) by a rogue umbrella caught in a gale on a deserted beach? Tragic, but simply unlucky. Have we tried to ban umbrellas or jogging? If there is a finite risk of something happening then you must accept that - very occasionally - it will happen?