Even though it may seem difficult for Western countries to join BRICS in the future, there may appear a time when countries will try to join. Especially when their own interests will be seriously affected by the long-term dominance of United States, it is not unusual for them to join BRICS to protect their national interests, where it is easier to expand trade. In the meantime, we know that there are some countries like France and Germany that have good relations with China. If they adopt such a policy in the future to protect their own interests, it will be realistic. Especially if BRICS works successfully in circulating their own currency against the dollar, then some countries in Europe will definitely try to free of the American regime.
That is very unlikely if not impossible, as they depend on the US not only economically but also militarily. The EU has been almost completely dependent on the US since World War II and to put it bluntly, the US has turned the EU into its lackey. Their relationship is no longer one of alliance and mutually beneficial cooperation in the literal sense. Therefore, the possibility of the EU leaving the US is unlikely.
Moreover, they also have their own calculations, what guarantees that Russia and China will welcome them with open arms? They would be in a dilemma if Russia and China did not accept them at that time, so they would consider carefully if they intended to move away from the US.