I think that goes back to identifying adversaries. It's unlikely a criminal would attempt to cut off fingers for access to a random smartphone.

If you have secrets that are very valuable, you are outside the standard use case, and should probably use more advanced authentication.

I figure a criminal would just rather have you change and disable your password altogether, it takes much less time that way and it's easier to do rather than dealing with carrying around a bloody finger.

I'd probably just unlock it for them if they had my phone and my person in their possession

If you're keeping information on your phone that may tempt adversaries to cut your fingers off, you may want to rethink some things.

Have there been any reports of this actually happening in the past year?

Touch ID only works for a minute or two after your finger is cut off. Touch ID reads the blood vessels, not the fingerprint, meaning it only works for a minute or two after the finger is cut off.

I'm not really interested in being the one on whom someone learns that, though.

Very few thieves would go that far, and they would almost certainly give you an opportunity to unlock the phone for them instead. Most people would take that opportunity.

Exactly. It doesn't matter how strong your password is when there is a gun in your face.

Ideally, you will have one that is easy to remember.

Er, that's just not true, as proven over and over by people fooling Touch ID with images of a fingerprint.