Earlier quoted context omitted.
>Bob exchanges currency B for currency A with Alice. Alice now holds some currency B There is no reason for Alice to want B, a less secure money than A which they already hold. If Alice were perfect mathematician, this exchange would never occur.
I'm failing to see your point. I do not know why, nether don't care, why Alice wants to buy currency B. Maybe she just wanted to buy some coffee from Charlie, and she was comfortable that her transaction didn't burn thousands of dollars of electricity to verify it? Maybe it was early in the morning? Faster block times means she'll get her coffee sooner. If that was the reason, let's hope she was satisfied with her co…
If a transaction does not do this, it can be guaranteed to be insecure.
>Faster block times means she'll get her coffee sooner
Maybe. Or maybe the merchant doesn't get paid for coffee at all (because faster blocktime means more orphans)!