Unfortunately, with the "Arena" style of these tracks, crashes like this will continue to happen regardless, and the crowds will always be in danger of a car breaking through the nets. There's no run off or way to catch the cars on the banks. So an out of control car will always be sent back through the pack compounding the crash.
The only way I can see to improve audience safety is to put a second car catcher between the stands and the wall. That way any car that breaks through the first will not have the energy to break through the second.
But honestly, the worst place you can stand in a race is the outside of a turn. Any rally fan will tell you that. But short of cutting the stands by 2/3rds and putting them in-field, there's nothing that can really be done about that.
The only thing I can think of is to remove the restrictor plates and run the road course. That should string the field out and keep the speeds down.