It also depends on what you are buying.. if you are buying a house, then the house is not moving, so if the payment is defective, then the title of the house would not have had validly transferred, and maybe the person cannot move in until the transaction is confirmed. I am not disagreeing with your points, and sure private parties can decide how many confirmations they need, and from my perspective, it is not too likely that the chain is getting rolled back so 1 confirmation is likely good enough even in large transactions... but yeah, so far in my bitcoin life, I have not tended to have transactions that are greater than $10k.. at least not so far.
I agree with your supporting point that is valid and to be honest, I did not think about that if you did not enlighten me on this point.
Value of a deal is important but it is not the only one, and your point contribute other factors for consideration too. By having multiple factors for careful consideration, people would have more clearer ideas on what they should do, and it will certainly increase their carefulness at somewhat level. Fortunately, by having more knowledge, more carefulness, it will only bring more careful and better practice that guarantees better result in the end.