This mitigation helps until the attackers stop confining themselves to the supply pins. You can do voltage fault injection through any exposed metal (or metal that can be made exposed), and the attacker can be entirely happy with attacks where they don't actually pull the voltage plane down, but instead just make some specific circuit a bit iffy by injecting an opposite voltage at some specific point and with low enough current that it's undetectable when looking at the chip as whole.

Approaches such as yours do make the attack require a lot more skill to accomplish.