Great article. I thought it was interesting that you shied away from the route of choosing a pistol caliber carbine. Although you give your reasons for that, it seems to me like being able to share ammo between your carbine and your sidearm would be a pretty huge benefit. I know in a situation where you would need a bug-out gun it would be foolish to hope for "ideal" conditions but it seems like a pistol caliber carbine would suffice in *most* situations.
Keep in mind that when I say "pistol caliber" I mean what a normal, rational, sentient human being would consider a pistol caliber, not the 7.62x39, 5.56, .223, 6.5 Grendel, 6.8SPC calibers that the ATF considers to be pistol calibers now. Even with a measly 9mm carbine, in my experience the rounds have enough oomph to get out there a pretty good distance, remain accurate and still pack a good punch. I would think that pistol calibers would cover most of your 100-200 yard range and still be very good for close quarters as well. With the aforementioned reclassification of (formerly) rifle caliber carbines, many good penetrating rounds will become scarce. Granted they can get out there farther and possibly faster but, in many cases, the projectiles will be carrying less weight with them than a pistol projectile and will deform just as much, if not more, upon impact.
The article also brings up barrel length considerations for concealability. When opting for a shorter barrel, would you not also lose much of the extra power you'd initially gain from starting off with the rifle caliber? Everyone's situation would no doubt be different, but I think that the ability to share ammo with a pistol, the lighter weight, the versatility and the extra power you'd gain from a longer barrel would really be strong arguments for pistol caliber.
You make a really good point about the reliability of a piston system, too. If you're forced to be on the move or living on a mountain with Jed and the boys when the Russians start parachuting in, you may not have time or access to supplies for proper cleaning and maintenance supplies.
Even with all my advocating of the benefits of pistol caliber carbines, I'm not really that big of a fan of them. I guess I've never really given that much consideration to what a good bug-out carbine would be beyond maybe the briefest of thoughts. Something like a SCAR or HK416 might make a good one. A good policy might just be to train and be versatile with a variety of systems so that should the Russians, Chinese and Cubans start falling out of the sky you can make do with what is on hand.