Castro and Khrushchev were feeling very positive towards JFK in the period just before he was assassinated. Castro never wanted hostile relations with the US in the first place (which makes sense for a very small country positioned just off the coast of the US). Likewise the USSR never wanted to just annihilate the US and in fact one of the causes of the Sino-Soviet split was that Khrushchev was interested in peaceful co-existence with the US. My own theory is that the Cuban Missile Crisis had a profound impact on JFK and Khrushchev. These were the only two men in human history who were both just a button push away from potentially destroying humanity. I think they both stood on the brink together and left that conflict with an understanding that things couldn’t go on like they had up to that point.
The idea that Cuba or the USSR had anything to do with assassinating JFK is absurd, IMO.
In addition to what others have said, you have to run very large, very persistent trade deficits for your currency to become a reserve currency (in addition to obviously needing to be a highly stable currency). Of the only currency issuers that could potentially fill reserve currency role (US, EU, China), only the US is interested in persistent trade deficits in order to pump the globe full of their currency.