Choices, choices!

1) Use only 3 keys and take a lesser chest while completing the quest.

2) Use 4 keys and get the best chest while idnoring the quest.

3) Build a better team so you can 3 key the CB (or 4 key with a rare), complete the quest, and actually progress in the game.

The idea that these quests should be taken away seems a bit myopic.  Basically you're saying if you can't do it, then no one else should have the opportunity to work toward a lego tome every 30 days.

No that's not what I'm saying, I'm saying a quest should be a foreward step, beat a better team, win a faction stage without losing a champ - these are not forward steps they are regressive - and thanks for the 'advice'

Actually, quest should require use "rare, or epic, or legendary champ" instead of limiting only to rare.

They are "advanced" quests.

For older, advanced accounts they become rather simple to complete. For newer less developed accounts they can be a pita.

In time none of them will give you problems.

As Evilized said, they're advanced for a reason. While many of the quests can be completed by mid-level+ accounts, there are tougher ones, like the one you mentioned, that are for players who have gotten to a point where they want to find a challenge again. 

It's impossible for me to complete the "Use 5 silver keys in the doom tower" I'm blocked since many many days and i receive that quest every day. 

you cant reach even the first boss on floor 10?