I thought the same too. Rule of two makes it less impressive since it implies it’s some key thing every Sith has to do and it’s expected. Being an anomaly as darklordoftech mentioned is far more interesting and there’s more impact to what Vader says. As if Vader has been loathing his master for a long time but had no means to overthrow him. Then comes his son and Vader has a “Ah yes, there’s finally hope in the galaxy after all!” moment. His aim was to restore order to the galaxy rather than have an apprentice for the sake of having one. I like that concept better imo.