Viscount Nelson only had a portion of the British fleet with him at Trafalgar so total defeat here would not have presented the French with any additional benefit. It certainly may have cut a few years off the Napoleonic Wars, which would have been a boon to the various European countries being economically devastated by the continuation of Napoleon's 'continental system'.
By 1821 Napoleonic Europe was coming apart at the seams and Napoleon's flight into exile did little to calm the revolutionary forces that reshaped the continent.