>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 coffee.
I also do not care why currency B got so popular, but I suspect it may have been because of the coffee ICO ;-)
Oh, and then there were cryptocurrencies C, D and F too, but that's a different thought experiment.
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.