We're not talking about a simple hard surface. We're talking about a handgun inside a bag filled with other miscellaneous items which is then jostled and pushed around.

You could argue the chance of an accidental discharge is low, but it's certainly meaningful enough to warrant caution. All the more so given the consequences of accidental discharge inside an airplane cabin.

All standard carry models I'm familiar with either have a hard trigger pull, or have a manual safety. I'll grant you that someone carrying the latter had better have it in a holster or some arrangement where the safety can't get pushed off ... but then again, if this is how they normally carry, e.g. a normal purse, if they don't do that they're always in danger of a negligent discharge. And there are modified guns out there with unreasonably light trigger pulls, but the same considerations above for protecting a manual safety hold.

So I'd say "very low", while agreeing that it's not zero.

BTW, it's almost never an accidental discharge, almost always a negligent one, and in this hypothetical case most certainly so.