For a spell with no target selection, there is already a sort of “cancel button” in the sense that you can choose not to cast the spell. Since that is the only decision involved, you can make your choice then move on.
For a spell with target selection, sometimes you make a mistake and try to cast, then notice after casting that a gem wasn’t where you thought, or you remember a troop has a trait that makes the spell useless. Hence the desire for this button. For these spells, you have to make two decisions, but you don’t have easy access to the information needed for the 2nd decision at all phases of casting.
But I agree, it’s likely implementing this feature will break several more troops and cause server downtime for several days, so we should probably just stop asking the devs to add things to the game.