Is it really that easy to issue a new unified currency among member countries of a bloc that will always remain subject to change? Who (which entity) will issue the currency? On what basis will its value be determined? How will the negative effects of the new currency on the local currencies of brics members be addressed? The only bloc in history that has succeeded in issuing a unified currency is the EU, and we can see the negative consequences of this on European economies. It has also failed to reduce dependence on the dollar.
It will definitely not be easy. But euro is a good example. It was established among tiny countries that were at war with each other for over a century (2 biggest ones they call WW1 and WW2) and yet they managed to create euro amongst themselves.
I dare say BRICS currency would be easier than that because first of all these countries were never really at war with each other at the level and it also doesn't need to be used as each nation's legal tender, it could be a currency for international trades only.
We have to wait and see where it goes though, at this point we are only speculating as we watch the progress...
For example, China, the largest holder of US bonds in the world,
China used to be the biggest bag-holder of US bonds holding IIRC $1.6 trillion of US debt. They've been dumping it slowly but surely though. Pretty much starting the dump since around 2014 (when WW5 entered a new phase). Today they are bag-holding around $0.75 trillion and are ranked third among the bag-holders (Japan is the first with $1.1 trillion, UK is second with $0.807 trillion bonds).
P.S. One of the reasons why FED doesn't doesn't reduce the interest rate is to prevent this dump!