I hope people will be more sympathetic to you in a context where you don’t excel than you are to people who don’t live up to your expectations.
I’ve fat-fingered a spell or two while playing on my phone. Hit the wrong troop, then hit cast while trying to back out. There’s always going to be a point where it’s too late to take it back, but I don’t see why the interval between pushing cast and selecting a target would be less reasonable than between picking a troop and hitting cast.
But perhaps you’d like to suggest that the cast screen be removed entirely, with spells being cast the moment you select a troop, so that players are required to memorize what all the spells do, and keep track of the relevant numbers outside the game. It would speed up battles a little for hardcore players, make things a little more convenient for them. But that’s an argument against it, isn’t it? You seem awfully opposed to convenience.
Part of game design is determining which skills you actually want to affect the player’s performance, and which shouldn’t be relevant. This isn’t a game that revolves around manual dexterity or reaction time. It doesn’t require the player to memorize abilities, or do a lot of math. The game provides that information, even during battles. Allowing players to back out of spell before selecting a target isn’t going to make them win games they’re not otherwise prepared or skillful enough to win. It just removes a little frustration, and minimizing frustration is one of the key components of interface design.