Obviously it is easier for YC to manage its founders and investments if they all embrace a common legal framework. Being a YC company does not necessarily mean your operations are based in the US, only that you incorporate there.
To respond to some of your questions:
> The president has undermined the rule of law....
My understanding is that the actions of the president or any other official can be successfully challenged in court. He doesn't have the last word
> Immigration policy - you would find it increasingly difficult to recruit foreign staff and could be stigmatised for hiring from outside the US
Not an issue if you operate outside the US. If you operate within the US, you deal with them the same way as your peers.
> Pressure to be an 'all American company' ...
Same answer as stated above
> Risk of war with the world
Nobody is going to war anytime soon. Trump is a fan of theatrics. As recent events have shown, more mature counsel will have its say.
But on a related note, I have always wondered - how do taxes work for YC companies incorporated in the US with operations abroad? Do they pay taxes both countries? This must impact their profitability and chances for survival in their growth stage