By that argument, William the Conqueror was just a general who killed the king and established himself as dictator.
Yes, but William the Conqueror found a royal line that, collaterally at least, still reigns in Great Britain almost 1,000 years later. After all, even though the current BRF is not the most senior line descending from Charles I Stuart, they descend from James I who, in turn, descends from Henry VII Tudor, who is himself a descendant of Henri II Plantagenet, who then descends from William the Conqueror. A similar argument could be made regarding the Orléans pretenders to the French throne for example (who descend from Louis XIII and, through him, collaterally all the way back to Hugh Capet).
Comparing those centuries-old royal lines to the one-term "empire" of Napoleon is ridiculous.