Magnetic tankbag? Take it with you (man bag) when you get off.
Oh, yes, I'd go magnetic or some other quick-detach method, surely.
I just don't always take the same luggage along on the bike. Heck, currently the most-used piece of "motorcycle luggage" is a big lunchbox cooler with a rigid inner shell that happens to fit the rear seat almost perfectly.
Luckily it matches the bike's color and doesn't look too terribly silly under the bungee net behind me.
But sometimes I load up a backpack which I may or may not put on the bike instead of on me.
And sometimes I just use the installed saddlebags, but I know that in a wreck they're likely to be sacrificed - and I don't want to be stranded, injured, and unarmed ... particularly if billy-bob ran me off the road in his truck-o-saurus
Plus with ANY bag I'd be taking the gun off and putting it on - I don't like doing that in the privacy of my car, let alone standing out in the open. The gun is safer than anywhere else on my person in a proper holster, monkeying around with it in between bag holster and belt holster doesn't appeal to me, and neither does a crappy holster that fits into a bag, I like my SuperTuck but I don't like storing it in limited space and I don't like putting the whole damn holster on in public, my carry methods are secure but not quick-on or quick-off.