If Trump were to be President the rest of the world would return to doing what they did when Bush was in the White House: work around the US in alternative groupings. The EU would resume a more leading role as others looked to it for sanity and stability. Russia / China would continue their partnership (entered into when Bush was elected in 2000) and that partnership would also gather friends and perhaps even allies. We've seen this before up to 2008.
Trump is much more aggressive and unstable than Bush, with less personal insight.
It's not the same situation as 2008.
Of course not....but the response would be similar because others have no other options but to isolate themselves as much as possible from the risks Trump imposes. IT's the same response any room full of people employs when someone in the room is too annoying to be tolerated: they ignore him as much as possible and limit - as much as possible - any engagement they can't avoid. This approach works daily and scales from creches to global leaders summits.
Donald is still in the creche ...