USA's share of global GDP is indeed declining, but that doesn't mean the BRICS form a coherent bloc. Rivalry between India and China is likely to be a defining geopolitical factor in Asia for the rest of the century, with Russia reduced to a peripheral role. And neither India nor South Africa are opponents of the US really, more like allies.
A multipolar world is emerging, and in fact already exists, but surely this means that an anti-imperialist, internationalist perspective now means decentering the US from your analysis. It is short sighted to celebrate the decline of US empire if it merely means the emergence of new, potentially worse Empires. Neither Chinese fascism, Russian oligarchy, nor India with their caste system look likely to bring about much improvement over US dominance.
This will probably be dismissed as a very woolly and naive liberal idea, but supporting the formation of EU-like regional blocs that prevent a single power from dominating seems a more realistic anti-imperialist idea than cheering for India or China or Russia to replace US dominance. Closer integration of the African Union, ASEAN, and some Latin American grouping - maybe some kind of relaunch of ALBA - is a good way to prevent them being captured by a comprador class - or invaded - under a new empire.
Some of the people who are inevitably going to sneer at this as liberal for mentioning the EU in a positive light were likely also cheering for ALBA as a way for Latin America to break from US influence 10 years ago, but it is essentially the same principle. The problem is these people are still stuck in 20th Century thinking and overrate the power of European countries, but the fact is that Europe is more an object than a subject of history these days, and without the EU we would be even more so - simply becoming client states of larger, predatory powers seeking to divide and rule. PwC predicted that by 2050 the only European states about the top 10 economies by size will be Germany and the UK, at number 9 and 10 respectively. And they are likely wrong about the UK.