Good thread and a remarkable lack of keyboard warrior's.
How I would react would depend on the situation.
If I see someone messing with my bike I will automatically charge at them and shout, there is no thought just instinct.
Last time I was naked and chasing the fuckers down the street.
A bike jacking is different though, weapons involved I would give it up, no weapons involved I would probably go down fighting (armoured gloves hurt)
Wife on the back give it up.
Fortunately no one has ever tried to take my bike from me, I find looking like I do most people leave me alone to do my own thing anyway.