In the US the final frontier is radicalized neo-confederate theocratic white Christian nationalist rural America that threatens the system's democratic legitimacy through institutionalized minority rule through an unrepresentative, rural-tilted senate and constitutional immobility. Throw in no term limits on the politicians in robes Supreme Court for good measure. If the American empire implodes, it won't be because of the drain of treasure and blood on its Baltic or Indo-Pacific frontier, but from the institutional rot within.

That said, of the last three imperial men left standing in our age of empire demolition--China, Russia and the US--the US is the least likely to implode first. China is the Blackberry of empire, Russia the Nokia and the USA's informal alliance network planetary empire is the iPhone of empire.