A BRICS currency has never been more important than it is now. The foundation of the USD as a reserve currency is based on its usage in the trade of oil. With Saudi Arabia, UAE, and Iran (oil producers) and Egypt (controlling the Suez) joining BRICS, there's a real threat to US currency dominance.
If the euro doesn't really threaten US as a reserve currency, a BRICS one certainly won't. As economies diversify and become more intersectional, as has happened since ww2, the currency fluctuations are less, with increased trade. Then currency used matters less and risk is lower for both sides when non us currency is used. So, I think dollar as a currency reserve will naturally reduce over time but it's not disappearing any time soon.