the thief is normally just an opportunist for a quick smash and grab imo
A camera thief will generally not be an opportunist - if you leave opportunity in the way of a thief then they will take it clearly.
However - what the thief will look for is a person who is carrying a large bag (like a large camera bag), or who is out with their children, or with their family.
They will not usually use a knife, but more likely a pair of industrial scissors (this mainly in case they should be stopped for whatever reason).
The cut takes less than a second, and because you are with your family, or with a child, or have a large bag etc - you are not likely to give chase.
The principle is employed to good effect in London, where many tourists wear "bum bags", or have their cameras and mobiles hanging around their necks.
The professional thief will look for a DSLR - something that is easy to sell on - don't operate under the impression that it is an opportunistic thing, as it is a very lucrative business and the chance of being caught - either physically by person being mugged, or the investigatively by the police is very low.
Don't walk around with your camera on display, or if it is on display then have it on your front with the neck strap out of sight. If you must have the camera on your back or side then use the metallic reinforced straps. It's a sad reflection of things unfortunately but any DSLR is an easy £50.