Contest mode raids and now the contest dungeon is the only really challenge for advanced players. But it is a once-in-a-lifetime 48h event, which is not possible to attend or fully attend for everyone due to several reasons (work, family, sickness, lag of a fireteam etc.). I see no reason, why it shoudn't be in the game permanently, to give everyone the chance to beat it. Just keep the exact conditions from day-one, incl. locked items, so it would not be unfair to people who had the opportunity to clear it early.