I appreciate the sentiment behind this line of thought, but there is always an inherent danger in having to simply "make it up," particularly as it relates to a universal capacity to disrupt casters. If the made up rule is effective and consistent in any capacity, it becomes a de facto strategy to shut down mid-to-high level casters.
Given your scenario above, a savvy party would gladly keep a rogue around (perhaps with Expertise in Arcana) to constantly keep attacks readied when facing a potent spellcaster. It's not difficult to find a way for the rogue to obtain advantage, so there's often going to be sneak attack damage added to any readied attack. If this same strategy was applied against the PCs, I doubt it'd be enthusiastically embraced by the players.
