As much as I am not particularly looking to coddle 5e PCs more, I feel like this is an obscure, counterintuitive, "gotcha" rule that primarily just serves to make the rules more needlessly complicated and byzantine, and to let a few of us rules professors impress everyone with our erudition (and everyone is not actually particularly impressed). I get what the designers are going for with it, but I just don't think it justifies the additional wrinkle of complexity.
Also a readied action is already a time when the player has sacrificed definitely attempting something with their action in favor of maybe attempting something with their action and reaction if a predicted circumstance arises, and often the readied action is only made necessary because of the artificial construct of the turn system (not so much the situation in question, where the nature of the enemies tactics require acting in a narrow window, but often you're just holding an action because your turn came up before the target of your action's). The player has already made an action economy sacrifice, so making them also lose any spell they are concentrating on seems needlessly punitive. But once again, even if it is a fair and balanced cost for the awesome opportunity to cast a spell out of turn order, I think it's just a needless complication of having an additional obscure rule that doesn't really add much to the experience.
So yeah, I wouldn't play with and effectively haven't played with this rule. I think it's poor game design, I wouldn't impose it as a DM, and nobody I play with is likely to know it or be thankful to hear my rattle it off at them when I'm a player.
Quitting a campaign over a rule that comes up that rarely is pretty wild though.
I disagree with all of your points.
This is not a gotcha rule.
It is an essential rule used to force spell casters to manage resources and intentional spellcasting. Especially in later levels when players are utilizing higher level casters and higher level spell effects.
What you are arguing is that a wizard, concentrating on a 6th level control spell should not have to lose concentration on that spell to ready a 7th level AoE spell to cast as a reaction, because that would be obscure, counterintuitive, and a "gotcha" rule.
I wholeheartedly disagree.
Casters should always be thinking about their spells, their effects, and what is needed in all types of encounters to provide the highest chance of success for the party. This rule pairs down a caster's power, and makes them have to think about the second, and perhaps third, order effects of trying to manage the power of concentration spells and reaction cast instantaneous spells.
No one has mentioned this, but the player purposefully gave the PC an 8 Dexterity Ability Score, because the player thought it was an "optimized" build decision. All this player did was learn that min/maxing is not always the same thing as optimizing, especially when their desired style of play in combat came into direct conflict with their choices made during character creation.
Enforcing a rule that jams up a players' poor decisions made during character creation does not make the rule 'bad'. In this instance, it highlighted the poor choices made by the player. By level seven, this player could have made different choices to bridge the gap between their abysmal Dexterity ability score and ranged combat challenges and choose not to.
The player incorrectly thought that he would be able to use True Strike to cover the deficiency and learned otherwise.
This is not the fault of the rules.
This is the fault of the shortsighted player.
The player did not evaluate all of the necessary rules before making this decision, and therefore did not build an 'optimized' character. Learning what works and what doesn't in game is part of playing the game. This player just didn't like the lesson learned, and rather than overcome the challenge, quit. Doesn't sound to me like the player is cut out for roleplaying games.
