There is no US plan to topple Assad, there never has been, and they have stated this openly on more than one occasion. They would like to see the fighting continue for an extended period, and eventually see a negotiated settlement that would leave Bashar in power over a redefined territory in a divided Syria.
Russia has much more to lose by its involvement in Syria than the US has to lose by Russia’s involvement, and it does nothing but guarantee a prolonged conflict, which the US wants, and Israel wants.