Look there is absolutely no debate that the US post-WW2 has been a force for good _in Europe_. They helped us rebuild, overcome communism and reunite under EU flag. Every single European owes a lot to the effort Americans put in to support us. And yes, I know they also did that because it helps them but you can't deny they've basically kickstarted every country that is now in the OECD. That generation of Americans will go down in history as some of the most influential humans ever. On par with Napoleon spreading the metric system and trade, the UK spreading the industrial revolution etc...
But they are also, really, unpredictable lately. George W. Bush was reelected after(!) starting 2 wars. Donald Trump is...well... unpredictable. And I have yet to see him be ousted in 2020. Let alone what the next Donald Trump looks like.
Imagine if someone came along with the same ideas who actually knows how to get stuff done in Washington.
It's just better to be self-sufficient in anything relating to critical infrastructure. As the Americans themselves say: hope for the best, prepare for the worst.