I still don't understand your disagreement with me about the opportunity cost of using the ready action when there is nothing else available to do with your action. If there is "nothing effective" to do with your action besides take the ready action then the cost of taking the ready action is "nothing effective" and I don't rate that as high. If you/your DM allows ready actions outside of combat, which generally would be the case for a"if the other party attacks us" scenario, then the cost is still "nothing effective" since there isn't an action economy outside of combat.
Okay, the argument I'm making is: If you'd only use a readied action if you have nothing else you
can do, that implies that readying is, in general, never better than
any other action. In which case, why even have it? It's useless.
If readying can do a thing which you can
only achieve by readied actions, that makes it more likely that you might actually give up a
useful action to get it. And that would make the rule less of a waste of space, and more of an actual tactical choice available to players.
So my point is, if you see players never ever using ready unless they've got no other ideas, then it's not that the opportunity cost is low; it's that the opportunity cost of using it
at any other time is so high that you never see it paid.
My mistake, I misread that you were attempting the target the actual concentration of the spell caster with your dispel. But I did address it later as you've found. I would not say concentration itself counts as a magical effect under the current rules.
Concentration doesn't have to be a magical effect in and of itself. The spell you are concentrating on is itself a "magical effect". That magical effect has a duration (while you are concentrating on it), and in general you can use dispel to dispel a "magical effect". The only usual exceptions are that you can't try to target the casting of the spell (that's counterspell) and that you can't dispel instantaneous spells after they have taken effect. But while someone's holding the spell for a readied action, it hasn't taken effect yet, and they are concentrating on it, not casting.
We were discussing the viability of breaking down the cast a spell action into casts a spell and resolution to which you replied therefore I don't think my response was off base.
The readied action rules clearly distinguish between "casting the spell" and "releasing the magic", with the later being when the spell's effects would get resolved.
Off topic: Line item quoting is going to makes this discussion difficult especially if when you comment on things that are continued later in the same quoted post. Directly quoting someone heavily implies you are responding to their points but I should have said "we/" are going off the point as it was just a phrase I was using to redirect my own post not a judgement on yours.
Line item quoting is the only way to have any idea what specific comments a given remark is in response to.