The legendary tasks are serving 2 purposes here:
1. If a guild is rolling in way more money than 16 mil every week (that's 4-5 top guilds really), they can do a couple extra tasks with all that gold. Maybe a few more if they're feeling particularly hardcore.
2. It's an extra outlet for players needing to make up their contributions in the other dozen guilds who finish their tasks... in these cases, if 1 extra task gets done it's a bonus
What the legendary tasks were NEVER intended to be was a return to the old system where top guilds were pulling in unsustainable amounts of gems & keys every week. That was eventually going to kill the game, and in the last 6 weeks we've seem some great results from that ,with way fewer players leaving in the mid-game, and instead joining guilds and getting more rewards.