Are we assuming the Qing are completely knocked out and can't rebound? It'd be a tough ask after losing the capital, but probably not entirely impossible.
I've always sort of envisaged any Taiping victory as degenerating rather quickly into an earlier warlord era, given Xiuquan's lack of effective leadership and control over his generals (see: the Tianjing Incident), the potential for all the other revolts to still bubble along, localised resistance to the Taiping, and the existence of previously Qing-affiliated forces who now have no one to answer to etc.
I imagine the western response would be very, very important.