Power creep is the tendency of games to slowly phase out old content to keep players chasing the new content. As new content gets introduced, it needs to be at least on par with current content in order for existing players to want to chase it. If you really want to encourage players to play it, the new gear is better than existing gear. So the end game players constantly have new content to engage with, so they don’t get bored and quit playing.

But this means that every single update increases the power cap just a little bit. As new gear, abilities, etc get introduced, players are able to get more and more powerful. But this means old content becomes easier and easier to complete, because it’s scaled for the content that existed when it was introduced. And old content can’t be rebalanced for the current content, because then new players will have an impossible barrier to entry; You still need to keep old content available and properly scaled, so new players can still break into the current content. So for instance, bosses that used to require four players to beat now only require two, or can even be solo’ed. Because the boss was scaled to be beatable with old gear, and new gear is stronger.