Given the ongoing split between US and EU it's no longer inconceivable that in 20-50 years trans-atlantic relationship deteriorates to the point of open warfare, sadly. Unfortunately, the brightest (or at least prudent) people aren't ruling either continent any longer.

I still find improbable that in 50 years there will be a single hegemonic “coalition” in Europe. So even if we will not be friends anymore US will have to deal with maybe 2-3 regional powers, not a single global one.

Your vision is unbelievable. The only way the us fights the eu is one or both sides are taken over by a complete dictatorship. The people on those countries would be diametrically opposed to this. Trump is not a dictator (regardless of his authoritarian preferences). Trump seems to be actively trying to knock the us out of cross country allegiances. I think whatever damage he causes, we'll be able to repair them over time.