Anyone who has used touch ID with wet fingers (happens all the time when I'm cooking) can attest to why this probably needs to be a tiny bit more than 3. 5 feels about right to me for this feature.

As an aside, if you are so paranoid that 5 is too much, or if this limitation exposes you to legitimate risk, you shouldn't be using Touch ID at all. Much harder to extract long password from your head than it is to force your finger on the sensor.