Not everyone has 8h+ a day, every day, to spend on this game. That's why older content becomes easier to access for those who are casual players and new content is hard for those hardcore players. New content will make old content obsolete in the long run yes but that's how the game will evolve.

Repeating the same content over and over because it is impossibly hard will push away all the casuals and will cut off 50% of the game's population.