Ready action gamebreaking situations

[The Crimson Binome]

Thus, spellcasters DO have the option of choosing to do something else once damage makes spellcasting impossible. Just as ANY character has the option of choosing to do something else once a readied action interrupts their turn and makes what they were GOING to do impossible.

That is simply not how the rules work.

Readying an Action

You can ready a standard action, a move action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, anytime before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character’s activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action.

The interrupted party can continue their action if they are able to do so - if you deal damage to try and disrupt a spell being cast, but they make their concentration check to continue casting, for example - but they don't get another chance to declare a new action. An action which fails, whether through an insufficient check result or a readied action or any other reason, is simply expended to zero effect.