I'll make a slightly different point than what I've seen so far (read all OPs posts and skimmed the rest)... sorry if I'm redundant.
I am of the personal opinion people should not rely upon their handgun as their primary defense tool... whether OC or concealed. Mindset is king in my book... so depend on your wits. As already mentioned, situational awareness is a big part of that. A handgun is just a tool to expand your options, but the use of motion and speech can also be very effective tools. Who knows, maybe this BG was entertaining himself by picking on "a little old man who thinks he's some kind of cowboy" and would not have actually stolen or used your gun. A sassy attitude with a few "get the he** off of me" may have put him off.
Something I learned way back in the day when I was very much into martial arts, is that most of your "tools" and techniques (whether they be offensive, defensive, evasion or flight) require a minimum distance between you and the "problem", and you might not be able to use them if somebody gets "inside" on you. Once that happens your in a grappling & wrestling match.
So part of interpreting your surroundings should be keeping an eye on how close people come to you. And if a conflict erupts with a person who is 2' or less from you, unless you can instantly spring into motion like cat, your in for a grappling, elbowing, head butting and rolling on the ground game. Good luck deploying pepper spray, when you already can't deploy your sidearm.
This is part of the reason I hate standing in lines.... and will gesture to let others go ahead of me, and then keep a more comfortable distance from them.
Based on the OP's self description, I doubt he's much up for a grappling match with a faster and stronger adversary, so OC debate aside, I don't think his situation is a good one for OC.
I tend to think about this in terms of personal space, which I happen to enjoy having more of, than less. And just because the government doesn't "profile" people, doesn't mean that I can't.... so if I can't keep outside of "smellin' distance" of some gang banger type.... I'm going to find another check out line, or continue my shopping or I'm just leaving my purchase on the shelf and walking out.
I'm curious if the store clerk or any of the other patrons were aware of what was going on. Shouting out "hey, stop that" or "get away from me" might have brought welcome assistance to you.... or may have dissuaded the BG from continuing.
The OP was caugth flat footed and unprepared... He didn't have his head in the game and was "shocked" that anyone would not be intimidated by his OC and automatically stand clear. Surprise! this young tough didn't bat an eyelash and was not at all intimidated in the least. So I'd say count your blessings and re-think everything, since the foundation of your personal defense strategy was just blatantly diregarded.
OBTW, if this just happened the other day, you may want to call the police. It's very possible, if not likely that the entire altercation is on video.