BRICS is the main threat for the USD supremacy since it's a group of country that is now more powerful in terms of economical strength. USA is already struggling against China in terms of economic war while BRICS is composed of strong country including China that controls the 23% of the global economy. This is enough to bring down US dollar as dominant currency while inflation will kill more it's buying power in long term. I think BRICS will be the next USD in the future.
This is not that simple, the US is a very powerful country with a lot of influence, so even if the BRICS nations are strong it will take more than that to displace the US dollar, after all there is no way to replace the US dollar without fundamentally changing the way the world economy works, and this means a huge economic crisis, so BRICS nations do not want this, and at best what they can do is to position themselves in order to take advantage when that crisis comes.
That's true, I do not think that USA will let those nations grow that easily and establish some instability in those nations to make sure that they are not growing all that much. Just look at Brazil, they tried their best hand on the elections there, the nation was in a lot of turmoil because of the elections and anyone who thinks that USA had nothing to do with it is funny, of course they played a part there.
So, they will do their best to keep those nations down as much as they can, and of course that will lead to USD keeping its power and never losing it. That's at least what they are trying and I am sure that they will achieve it one way or another as well. It is not that simple, but it will be possible after a while.