My thoughts on this:
I've owned:
18" 870 w/Mesa M4 Stock
Benelli M4
870P w/Surefire Foreend
Mossberg 590

I still own the 590, if for nothing else than I have quite a bit of 12g ammo and need something other than my Citori to shoot it.

My thought process on the shotgun is that it is a specialized tool. Breaching, hunting, "home defense", etc, and yet each of those applications likely needs to be set up differently to be effective in their specialization.

I feel like "end of the world" fantasies are pretty unrealistic. A good concealed pistol with a spare mag is probably going to be gold in those scenarios as it doesn't really matter if you are carrying a woodstocked 870 or an AR15, most folks arent gonna be pleased by the sight of it.

As far as a true do it all defensive weapon, I am hard pressed to see where the shotgun outshines the 14.5" AR15. Length, recoil, capacity, reloads, distance, speed, etc. Yes, the shotgun is devastating at close range, but it has many drawbacks compared to an AR.

So, with all this in mind, I decided to keep the 590 in my safe, and buy more training and ammo for pistol/carbine courses than buy another shotgun.