You are forgetting the capacitor is, well, a capacitor. In order to get it up to failure voltage you need to charge it up that high.

When the switch opens the voltage across the capacitor will not change instantaneously like the coil voltage does. Instead, the voltage spike presents itself across the resistor. The energy from the coil is then dissipated through the resistor and into the capacitor.

The capacitance then needs to be large enough to absorb the energy in the coil without exceeding it's rated voltage.

You are forgetting the capacitor is, well, a capacitor. In order to get it up to failure voltage you need to charge it up that high.

When the switch opens the voltage across the capacitor will not change instantaneously like the coil voltage does. Instead, the current in the coil passes trough the resistor to charge the capacitor. The voltage across the coil is therefor limited. The energy from the coil is then dissipated through the resistor and into the capacitor.

The capacitance then needs to be large enough to absorb the energy in the coil without exceeding it's rated voltage.