Is there any idiom or expression that would imply " It is hard to argue against such rationale"?

Solution 1:

All of these answers accurately describe how one would claim that their own argument has no room for dispute, but your question specifically mentions the speaker admitting the other side has a point. Therefore, I would suggest the following:

• "I can't argue with that!"

• "You got me there."

• "That's a good point."

• "I'll give you that."

Inflection is important with all of these, and it might help to add "Well," or "I guess" to the start of each of them.

Solution 2:

I believe using the word "touché", where you are affirming someone else's rationale rather than your own, would be appropriate. If I'm understanding the scenario this would be what Alex would say in response to James' irrefutable argument.