Is a magnetic reversal threat to life on Earth? Why do you say this?

No I don't think there'll be a major threat to life. (Except people who are addicted to their phones will be very sad that their phones won't work as well, there might not be any service any where, and they have to spend money and time to buy a new phone.) This is because the Earth's magnetic field is contained in the magnetosphere, which deflects many high energy particles. If the magnetosphere were to be interfered with, then there might be a threat to life on Earth.