The UCMJ is the Congressional Code of Military Criminal Law applicable to all military members worldwide. It should also apply to its Commander in Chief.
It could not possibly apply to the President. Who would be the President's commander, who is necessary to prefer charges? Who would be his Special Court Martial Convening Authority, who would refer charges and appoint the Art. 32 Investigating Officer? Who would be his General Court Martial Convening Authority, and what possible members would be eligible to sit on a court martial (CM) to judge their own commanding officer? Everyone necessary to conduct a CM would be ineligible to act on the case, including the military judge. With senior military members accused officers, these problems are easily addressed by their removal, temporary or permanent from command position, which can be done for any reason at all ("loss of confidence in ability to command"), but this only works because there is always someone senior to make that decision. Applying the UCMJ to the President could not possible work because there is no one senior to him in the chain of command to make the decisions necessary to even prefer charges.
I get the idea of no one being above the law. However, the UCMJ is both substantive and procedural and none of the procedures could apply to a President to make the substantive determinations of guilt or innocence. It just could not work.
All of the above sidetracking from the OP aside, the Demise of the US is neither inevitable nor is is imminent. Taking the really long view, the framework we have in place, under the Constitution, is plenty robust enough to deal with and adapt to any serious challenges. It may be ugly, slow, and not to every persons liking in dealing with issues that are more mundane like our budget issues, but the US, though not an unstoppable juggernaut in every single aspect, is in a different situation than countries like Greece, Italy, Portugal or Spain. Want evidence of this? After our credit rating was lowered, the value of Treasuries actually increased. Countries around the world still buy our debt and it is pretty much seen as the safest bet around the world by everyone. What people forget to take into account is that our economy is so strong (yes, I said it), that if we sneeze, the rest of the world catches a cold. In this respect, we are in essentially a zero sum game- one that does not seriously allow us to lose strength relative to other countries. If the US "falls apart" and all that debt that is owned by other countries loses value (and the other countries then lose proportionally). The question is not whether we are going to collapse altogether; it is whether the US will continue to be as strong as it has been in the past or could be if we pursued different policies. But, in a worst case scenario, we are not near the edge of "demise." Could things change in a hundred, two hundred, or five hundred years? Sure, I suppose so. But, just like the children of billionaires, the US is starting from such an advantaged position, it is hard to imagine not doing well in the future. Possible? Remotely, I guess. Likely? Hardly so.
That's my optimistic post of the day! Now, everyone, start planning for your New England Patriots victory celebration. I might be wrong about the last point; if the Giants do some magic, don't take that as sign of imminent US demise. Though, if you did, I would understand the sentiment!!!