All the players should be allowed to experience the story - once.

Instead of nerfing content for everyone, I think it would be better to nerf it only when there are new players doing it to allow them smooth progression.

Any subsequent run should be more difficult, as the instance is no longer needed for story progression and players should know the basics.

In my opinion it is better to scale the difficulty based on players in the group, rather than scaling it down for everybody just in case there would be someone new.